fix: Maven Toolchains grows unexpectedly (#534)

* fix: Maven Toolchains grows unexpectedly

On self-hosted runners toolchains.xml may survive multiple runs and unexpectedly
grow as a result of the toolchains setup simply appending the JDK definition
even if one with the same `type` and `provides.id` already exists.

Restructuring the parsing step and filtering the potentially existing list of
toolchain definitions prevents this and also fixes toolchain.xml files that
already contain duplicates.

Fixes #530

* fix: guard toolchain dedup and preserve existing root attributes

Address reviewer feedback on the Maven toolchains dedup logic:

- Treat jdk toolchains without a string `provides.id` as non-deduplicatable
  and use optional access when comparing ids, so partially-formed or
  nonstandard toolchains.xml files no longer crash setup.
- Preserve the existing `<toolchains>` root attributes (xmlns,
  schemaLocation, …) when present, falling back to the 1.1.0 defaults only
  for attributes the existing file is missing. This avoids silently
  rewriting user-managed metadata or changing the effective namespace.

Adds tests covering custom root attributes and id-less jdk toolchains, and
rebuilds dist/.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Bruno Borges <brborges@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

__tests__/toolchains.test.ts

@@ -143,7 +143,20 @@ describe('toolchains tests', () => {
         </toolchain>
       </toolchains>`;
     const result = `<?xml version="1.0"?>
-<toolchains>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
   <toolchain>
     <type>jdk</type>
     <provides>
@@ -155,6 +168,68 @@ describe('toolchains tests', () => {
       <jdkHome>/opt/jdk/sun/1.6</jdkHome>
     </configuration>
   </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('does not discard custom elements in existing toolchain definitions', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+        <toolchain>
+          <type>jdk</type>
+          <provides>
+            <version>1.6</version>
+            <vendor>Sun</vendor>
+            <id>sun_1.6</id>
+            <custom>foo</custom>
+          </provides>
+          <configuration>
+            <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+            <fooHome>/usr/local/bin/bash</fooHome>
+          </configuration>
+        </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
   <toolchain>
     <type>jdk</type>
     <provides>
@@ -166,6 +241,544 @@ describe('toolchains tests', () => {
       <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
     </configuration>
   </toolchain>
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>1.6</version>
+      <vendor>Sun</vendor>
+      <id>sun_1.6</id>
+      <custom>foo</custom>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+      <fooHome>/usr/local/bin/bash</fooHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('does not discard existing, custom toolchain definitions', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+        <toolchain>
+            <type>foo</type>
+            <provides>
+                <custom>baz</custom>
+            </provides>
+            <configuration>
+                <fooHome>/usr/local/bin/foo</fooHome>
+            </configuration>
+        </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+  <toolchain>
+    <type>foo</type>
+    <provides>
+      <custom>baz</custom>
+    </provides>
+    <configuration>
+      <fooHome>/usr/local/bin/foo</fooHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('does not duplicate existing toolchain definitions', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+      <toolchain>
+        <type>jdk</type>
+        <provides>
+          <version>17</version>
+          <vendor>Eclipse Temurin</vendor>
+          <id>temurin_17</id>
+        </provides>
+        <configuration>
+          <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+        </configuration>
+      </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('does not duplicate existing toolchain definitions if multiple exist', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+        <toolchain>
+          <type>jdk</type>
+          <provides>
+            <version>1.6</version>
+            <vendor>Sun</vendor>
+            <id>sun_1.6</id>
+          </provides>
+          <configuration>
+            <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+          </configuration>
+        </toolchain>
+      <toolchain>
+        <type>jdk</type>
+        <provides>
+          <version>17</version>
+          <vendor>Eclipse Temurin</vendor>
+          <id>temurin_17</id>
+        </provides>
+        <configuration>
+          <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+        </configuration>
+      </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>1.6</version>
+      <vendor>Sun</vendor>
+      <id>sun_1.6</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('handles an empty list of existing toolchains correctly', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('handles an empty existing toolchains.xml correctly', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = ``;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('preserves custom root attributes on existing toolchains.xml', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.0.0 http://maven.apache.org/xsd/toolchains-1.0.0.xsd">
+        <toolchain>
+          <type>jdk</type>
+          <provides>
+            <version>1.6</version>
+            <vendor>Sun</vendor>
+            <id>sun_1.6</id>
+          </provides>
+          <configuration>
+            <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+          </configuration>
+        </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.0.0 http://maven.apache.org/xsd/toolchains-1.0.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>1.6</version>
+      <vendor>Sun</vendor>
+      <id>sun_1.6</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+    </configuration>
+  </toolchain>
+</toolchains>`;
+
+    fs.mkdirSync(m2Dir, {recursive: true});
+    fs.writeFileSync(toolchainsFile, originalFile);
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+
+    await toolchains.createToolchainsSettings({
+      jdkInfo,
+      settingsDirectory: m2Dir,
+      overwriteSettings: true
+    });
+
+    expect(fs.existsSync(m2Dir)).toBe(true);
+    expect(fs.existsSync(toolchainsFile)).toBe(true);
+    expect(fs.readFileSync(toolchainsFile, 'utf-8')).toEqual(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    );
+    expect(
+      toolchains.generateToolchainDefinition(
+        originalFile,
+        jdkInfo.version,
+        jdkInfo.vendor,
+        jdkInfo.id,
+        jdkInfo.jdkHome
+      )
+    ).toEqual(result);
+  }, 100000);
+
+  it('keeps partially-formed jdk toolchains without an id instead of crashing', async () => {
+    const jdkInfo = {
+      version: '17',
+      vendor: 'Eclipse Temurin',
+      id: 'temurin_17',
+      jdkHome: '/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64'
+    };
+
+    const originalFile = `<toolchains>
+        <toolchain>
+          <type>jdk</type>
+          <provides>
+            <version>1.6</version>
+            <vendor>Sun</vendor>
+          </provides>
+          <configuration>
+            <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+          </configuration>
+        </toolchain>
+      </toolchains>`;
+    const result = `<?xml version="1.0"?>
+<toolchains xmlns="http://maven.apache.org/TOOLCHAINS/1.1.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd">
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>17</version>
+      <vendor>Eclipse Temurin</vendor>
+      <id>temurin_17</id>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/17.0.1-12/x64</jdkHome>
+    </configuration>
+  </toolchain>
+  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>1.6</version>
+      <vendor>Sun</vendor>
+    </provides>
+    <configuration>
+      <jdkHome>/opt/jdk/sun/1.6</jdkHome>
+    </configuration>
+  </toolchain>
 </toolchains>`;
 
     fs.mkdirSync(m2Dir, {recursive: true});

dist/setup/index.js

@@ -81506,46 +81506,66 @@ function createToolchainsSettings({ jdkInfo, settingsDirectory, overwriteSetting
 exports.createToolchainsSettings = createToolchainsSettings;
 // only exported for testing purposes
 function generateToolchainDefinition(original, version, vendor, id, jdkHome) {
-    let xmlObj;
+    let jsToolchains = [
+        {
+            type: 'jdk',
+            provides: {
+                version: `${version}`,
+                vendor: `${vendor}`,
+                id: `${id}`
+            },
+            configuration: {
+                jdkHome: `${jdkHome}`
+            }
+        }
+    ];
+    // default root attributes, used when the existing file does not declare its own
+    let rootAttributes = {
+        '@xmlns': 'http://maven.apache.org/TOOLCHAINS/1.1.0',
+        '@xmlns:xsi': 'http://www.w3.org/2001/XMLSchema-instance',
+        '@xsi:schemaLocation': 'http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd'
+    };
     if (original === null || original === void 0 ? void 0 : original.length) {
-        xmlObj = (0, xmlbuilder2_1.create)(original)
+        // convert existing toolchains into TS native objects for better handling
+        // xmlbuilder2 will convert the document into a `{toolchains: { toolchain: [] | {} }}` structure
+        // instead of the desired `toolchains: [{}]` one or simply `[{}]`
+        const jsObj = (0, xmlbuilder2_1.create)(original)
             .root()
-            .ele({
+            .toObject();
-            toolchain: {
+        if (jsObj.toolchains) {
-                type: 'jdk',
+            // preserve the existing root attributes (xmlns, schemaLocation, …) so we don't
-                provides: {
+            // silently rewrite user-managed metadata or change the effective XML namespace;
-                    version: `${version}`,
+            // xmlbuilder2 exposes attributes as `@`-prefixed keys on the element object
-                    vendor: `${vendor}`,
+            const existingAttributes = Object.fromEntries(Object.entries(jsObj.toolchains).filter(([key]) => key.startsWith('@')));
-                    id: `${id}`
+            // fall back to the defaults only for attributes the existing file is missing
-                },
+            rootAttributes = Object.assign(Object.assign({}, rootAttributes), existingAttributes);
-                configuration: {
+            if (jsObj.toolchains.toolchain) {
-                    jdkHome: `${jdkHome}`
+                // in case only a single child exists xmlbuilder2 will not create an array and using verbose = true equally doesn't work here
+                // See https://oozcitak.github.io/xmlbuilder2/serialization.html#js-object-and-map-serializers for details
+                if (Array.isArray(jsObj.toolchains.toolchain)) {
+                    jsToolchains.push(...jsObj.toolchains.toolchain);
+                }
+                else {
+                    jsToolchains.push(jsObj.toolchains.toolchain);
                 }
             }
+        }
+        // remove potential duplicates based on type & id (which should be a unique combination);
+        // self.findIndex will only return the first occurrence, ensuring duplicates are skipped
+        jsToolchains = jsToolchains.filter((value, index, self) => {
+            var _a;
+            // ensure non-jdk toolchains are kept in the results, we must not touch them because they belong to the user
+            return value.type !== 'jdk' ||
+                // keep toolchains that lack a usable string id (e.g. partially-formed user files);
+                // we cannot safely deduplicate them and must not crash while reading them
+                typeof ((_a = value.provides) === null || _a === void 0 ? void 0 : _a.id) !== 'string' ||
+                index ===
+                    self.findIndex(t => { var _a, _b; return t.type === value.type && ((_a = t.provides) === null || _a === void 0 ? void 0 : _a.id) === ((_b = value.provides) === null || _b === void 0 ? void 0 : _b.id); });
         });
     }
-    else
+    return (0, xmlbuilder2_1.create)({
-        xmlObj = (0, xmlbuilder2_1.create)({
+        toolchains: Object.assign(Object.assign({}, rootAttributes), { toolchain: jsToolchains })
-            toolchains: {
+    }).end({
-                '@xmlns': 'http://maven.apache.org/TOOLCHAINS/1.1.0',
-                '@xmlns:xsi': 'http://www.w3.org/2001/XMLSchema-instance',
-                '@xsi:schemaLocation': 'http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd',
-                toolchain: [
-                    {
-                        type: 'jdk',
-                        provides: {
-                            version: `${version}`,
-                            vendor: `${vendor}`,
-                            id: `${id}`
-                        },
-                        configuration: {
-                            jdkHome: `${jdkHome}`
-                        }
-                    }
-                ]
-            }
-        });
-    return xmlObj.end({
         format: 'xml',
         wellFormed: false,
         headless: false,

src/toolchains.ts

@@ -83,47 +83,76 @@ export function generateToolchainDefinition(
   id: string,
   jdkHome: string
 ) {
-  let xmlObj;
+  let jsToolchains: Toolchain[] = [
-  if (original?.length) {
+    {
-    xmlObj = xmlCreate(original)
+      type: 'jdk',
-      .root()
+      provides: {
-      .ele({
+        version: `${version}`,
-        toolchain: {
+        vendor: `${vendor}`,
-          type: 'jdk',
+        id: `${id}`
-          provides: {
+      },
-            version: `${version}`,
+      configuration: {
-            vendor: `${vendor}`,
+        jdkHome: `${jdkHome}`
-            id: `${id}`
-          },
-          configuration: {
-            jdkHome: `${jdkHome}`
-          }
-        }
-      });
-  } else
-    xmlObj = xmlCreate({
-      toolchains: {
-        '@xmlns': 'http://maven.apache.org/TOOLCHAINS/1.1.0',
-        '@xmlns:xsi': 'http://www.w3.org/2001/XMLSchema-instance',
-        '@xsi:schemaLocation':
-          'http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd',
-        toolchain: [
-          {
-            type: 'jdk',
-            provides: {
-              version: `${version}`,
-              vendor: `${vendor}`,
-              id: `${id}`
-            },
-            configuration: {
-              jdkHome: `${jdkHome}`
-            }
-          }
-        ]
       }
-    });
+    }
+  ];
+  // default root attributes, used when the existing file does not declare its own
+  let rootAttributes: Record<string, string> = {
+    '@xmlns': 'http://maven.apache.org/TOOLCHAINS/1.1.0',
+    '@xmlns:xsi': 'http://www.w3.org/2001/XMLSchema-instance',
+    '@xsi:schemaLocation':
+      'http://maven.apache.org/TOOLCHAINS/1.1.0 https://maven.apache.org/xsd/toolchains-1.1.0.xsd'
+  };
+  if (original?.length) {
+    // convert existing toolchains into TS native objects for better handling
+    // xmlbuilder2 will convert the document into a `{toolchains: { toolchain: [] | {} }}` structure
+    // instead of the desired `toolchains: [{}]` one or simply `[{}]`
+    const jsObj = xmlCreate(original)
+      .root()
+      .toObject() as unknown as ExtractedToolchains;
+    if (jsObj.toolchains) {
+      // preserve the existing root attributes (xmlns, schemaLocation, …) so we don't
+      // silently rewrite user-managed metadata or change the effective XML namespace;
+      // xmlbuilder2 exposes attributes as `@`-prefixed keys on the element object
+      const existingAttributes = Object.fromEntries(
+        Object.entries(jsObj.toolchains).filter(([key]) => key.startsWith('@'))
+      ) as Record<string, string>;
+      // fall back to the defaults only for attributes the existing file is missing
+      rootAttributes = {...rootAttributes, ...existingAttributes};
 
-  return xmlObj.end({
+      if (jsObj.toolchains.toolchain) {
+        // in case only a single child exists xmlbuilder2 will not create an array and using verbose = true equally doesn't work here
+        // See https://oozcitak.github.io/xmlbuilder2/serialization.html#js-object-and-map-serializers for details
+        if (Array.isArray(jsObj.toolchains.toolchain)) {
+          jsToolchains.push(...jsObj.toolchains.toolchain);
+        } else {
+          jsToolchains.push(jsObj.toolchains.toolchain);
+        }
+      }
+    }
+
+    // remove potential duplicates based on type & id (which should be a unique combination);
+    // self.findIndex will only return the first occurrence, ensuring duplicates are skipped
+    jsToolchains = jsToolchains.filter(
+      (value, index, self) =>
+        // ensure non-jdk toolchains are kept in the results, we must not touch them because they belong to the user
+        value.type !== 'jdk' ||
+        // keep toolchains that lack a usable string id (e.g. partially-formed user files);
+        // we cannot safely deduplicate them and must not crash while reading them
+        typeof value.provides?.id !== 'string' ||
+        index ===
+          self.findIndex(
+            t => t.type === value.type && t.provides?.id === value.provides?.id
+          )
+    );
+  }
+
+  return xmlCreate({
+    toolchains: {
+      ...rootAttributes,
+      toolchain: jsToolchains
+    }
+  }).end({
     format: 'xml',
     wellFormed: false,
     headless: false,
@@ -166,3 +195,24 @@ async function writeToolchainsFileToDisk(
     flag: 'w'
   });
 }
+
+interface ExtractedToolchains {
+  toolchains: {
+    // root attributes such as xmlns / schemaLocation are exposed as `@`-prefixed keys
+    [attribute: `@${string}`]: string;
+    toolchain?: Toolchain[] | Toolchain;
+  };
+}
+
+// Toolchain type definition according to Maven Toolchains XSD 1.1.0
+interface Toolchain {
+  type: string;
+  provides:
+    | {
+        version: string;
+        vendor: string;
+        id: string;
+      }
+    | any;
+  configuration: any;
+}