Fix build preset jobs handling and add cmake.parallelJobs fallback (#4731)

* Initial plan

* Fix CMake build preset jobs flag and parallelJobs setting inconsistency

- Fix buildArgs() to handle jobs: 0 (0 is falsy in JS but valid for cmake --parallel 0)
- Add fallbackJobs parameter to buildArgs() for cmake.parallelJobs setting fallback
- Pass config.numJobs as fallback in generateBuildCommandFromPreset()
- Fix same jobs: 0 issue in testArgs() for consistency
- Add unit tests for buildArgs() covering all edge cases

Co-authored-by: snehara99 <[email protected]>

* Use raw parallelJobs for preset fallback to preserve jobs: 0

Add numJobsForPreset getter that preserves 0 as-is (since CMake
natively handles --parallel 0) instead of converting to CPU count.
Use it in generateBuildCommandFromPreset() as the fallback.
Add test for fallbackJobs: 0 case.

Co-authored-by: snehara99 <[email protected]>

* Move cmake.parallelJobs to override settings table in docs

Since our PR uses cmake.parallelJobs as a fallback when the build preset
doesn't specify jobs, the documentation should reflect that it's no
longer completely ignored when presets are active.

Co-authored-by: snehara99 <[email protected]>

* Use -j instead of --parallel in preset buildArgs

Per reviewer preference, use the shorter -j flag instead of --parallel
when passing the jobs count to cmake --build. Both flags are equivalent
in CMake (cmake --build -j N == cmake --build --parallel N).

Co-authored-by: snehara99 <[email protected]>

* Use -j 0 for jobs==0 and --parallel N for jobs>0 in buildArgs

Per reviewer request: use the shorter -j flag only when the value is 0,
and use --parallel with the explicit count for positive values.

Co-authored-by: snehara99 <[email protected]>

* Move cmake.parallelJobs back to ignored settings table with fallback note

The setting doesn't override buildPresets.jobs - it's only used as a
fallback when jobs is not defined in the preset. Update the description
to clarify this behavior.

Co-authored-by: snehara99 <[email protected]>

* Use cmake.parallelJobs fallback when preset jobs is 0 or undefined

When buildPresets.jobs is 0, treat it like "not defined" and fall back
to cmake.parallelJobs. If cmake.parallelJobs is also not set, the
preset's own 0 value is preserved (producing -j 0).

Updated docs and tests to match.

Co-authored-by: snehara99 <[email protected]>

* Revert last 2 commits per reviewer request

Reverts the docs change (moving cmake.parallelJobs back to ignored table)
and the code change (using fallback when preset jobs is 0). Restores the
state from commit e907de6.

Co-authored-by: snehara99 <[email protected]>

* Revert docs to original state: cmake.parallelJobs in ignored table

Move cmake.parallelJobs back to the "Ignored settings" table where it
was originally, removing it from the "Override settings" table.

Co-authored-by: snehara99 <[email protected]>

* Changes before error encountered

Co-authored-by: snehara99 <[email protected]>

* Changes before error encountered

Co-authored-by: snehara99 <[email protected]>

* Fix tests to expect bare -j (no value) when jobs is 0

The code already pushes bare '-j' for jobs===0, but the tests still
expected '-j' followed by '0'. Updated tests to verify bare -j is
emitted without a numeric argument.

Co-authored-by: snehara99 <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: snehara99 <[email protected]>

Author: Copilot

CHANGELOG.md

@@ -35,6 +35,7 @@ Bug Fixes:
 - Add support for Visual Studio 2026 generator. [#4637](https://github.com/microsoft/vscode-cmake-tools/issues/4637)
 - Fix `$comment` not being accepted inside a cacheVariable object in CMake presets. [#4600](https://github.com/microsoft/vscode-cmake-tools/issues/4600)
 - Fix kits from `cmake.additionalKits` not being shown when `cmake.showSystemKits` is `false`. [#4651](https://github.com/microsoft/vscode-cmake-tools/issues/4651)
+- Fix how `jobs` is handled in build presets. Also update how `cmake.parallelJobs` is handled as a fallback when a build preset does not define `jobs`. [#4176](https://github.com/microsoft/vscode-cmake-tools/issues/4176)
 
 ## 1.22.28
 

src/config.ts

@@ -522,6 +522,13 @@ export class ConfigurationReader implements vscode.Disposable {
         }
     }
 
+    get numJobsForPreset(): number | undefined {
+        if (this.isDefaultValue("parallelJobs")) {
+            return undefined;
+        }
+        return this.parallelJobs;
+    }
+
     get numCTestJobs(): number {
         const ctestJobs = this.ctestParallelJobs;
         if (!ctestJobs) {

src/drivers/cmakeDriver.ts

@@ -1952,7 +1952,7 @@ export abstract class CMakeDriver implements vscode.Disposable {
         } else {
             buildPreset.__targets = buildPreset.targets;
         }
-        const args = preset.buildArgs(buildPreset, this.config.buildArgs, this.config.buildToolArgs);
+        const args = preset.buildArgs(buildPreset, this.config.buildArgs, this.config.buildToolArgs, this.config.numJobsForPreset);
         const initialEnvironment = EnvironmentUtils.create(buildPreset.environment);
         const build_env = await this.getCMakeBuildCommandEnvironment(initialEnvironment);
         const expanded_args_promises = args.map(async (value: string) => expand.expandString(value, { ...this.expansionOptions, envOverride: build_env }));

src/presets/preset.ts

@@ -2210,11 +2210,18 @@ export function configureArgs(preset: ConfigurePreset): string[] {
     return result;
 }
 
-export function buildArgs(preset: BuildPreset, tempOverrideArgs?: string[], tempOverrideBuildToolArgs?: string[]): string[] {
+export function buildArgs(preset: BuildPreset, tempOverrideArgs?: string[], tempOverrideBuildToolArgs?: string[], fallbackJobs?: number): string[] {
     const result: string[] = [];
 
     preset.__binaryDir && result.push('--build', preset.__binaryDir);
-    preset.jobs && result.push('--parallel', preset.jobs.toString());
+    const jobs = preset.jobs ?? fallbackJobs;
+    if (jobs !== undefined) {
+        if (jobs === 0) {
+            result.push('-j');
+        } else {
+            result.push('--parallel', jobs.toString());
+        }
+    }
     preset.configuration && result.push('--config', preset.configuration);
     preset.cleanFirst && result.push('--clean-first');
     preset.verbose && result.push('--verbose');
@@ -2290,7 +2297,7 @@ export function testArgs(preset: TestPreset): string[] {
     if (preset.execution) {
         preset.execution.stopOnFailure && result.push('--stop-on-failure');
         preset.execution.enableFailover && result.push('-F');
-        preset.execution.jobs && result.push('--parallel', preset.execution.jobs.toString());
+        (preset.execution.jobs !== undefined) && result.push('--parallel', preset.execution.jobs.toString());
         preset.execution.resourceSpecFile && result.push('--resource-spec-file', preset.execution.resourceSpecFile);
         preset.execution.testLoad && result.push('--test-load', preset.execution.testLoad.toString());
         preset.execution.showOnly && result.push('--show-only', preset.execution.showOnly);

test/unit-tests/presets/presets.test.ts

@@ -1,4 +1,4 @@
-import { Condition, configureArgs, evaluateCondition, getArchitecture, getToolset } from '@cmt/presets/preset';
+import { buildArgs, Condition, configureArgs, evaluateCondition, getArchitecture, getToolset } from '@cmt/presets/preset';
 import { expect } from '@test/util';
 import * as os from "os";
 
@@ -195,4 +195,83 @@ suite('Preset tests', () => {
         const args5 = configureArgs(preset5);
         expect(args5).to.deep.eq([]);
     });
+
+    test('buildArgs handles jobs: 0 correctly', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build',
+            jobs: 0
+        };
+        const args = buildArgs(preset);
+        expect(args).to.include('-j');
+        // -j should be bare (no value) — next arg should NOT be '0'
+        const idx = args.indexOf('-j');
+        expect(args[idx + 1]).to.not.eq('0');
+    });
+
+    test('buildArgs handles jobs: positive number', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build',
+            jobs: 8
+        };
+        const args = buildArgs(preset);
+        const idx = args.indexOf('--parallel');
+        expect(idx).to.be.greaterThan(-1);
+        expect(args[idx + 1]).to.eq('8');
+    });
+
+    test('buildArgs omits -j and --parallel when jobs is undefined and no fallback', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build'
+        };
+        const args = buildArgs(preset);
+        expect(args).to.not.include('-j');
+        expect(args).to.not.include('--parallel');
+    });
+
+    test('buildArgs uses fallbackJobs when jobs is undefined', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build'
+        };
+        const args = buildArgs(preset, undefined, undefined, 4);
+        const idx = args.indexOf('--parallel');
+        expect(idx).to.be.greaterThan(-1);
+        expect(args[idx + 1]).to.eq('4');
+    });
+
+    test('buildArgs prefers preset jobs over fallbackJobs', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build',
+            jobs: 2
+        };
+        const args = buildArgs(preset, undefined, undefined, 8);
+        const idx = args.indexOf('--parallel');
+        expect(idx).to.be.greaterThan(-1);
+        expect(args[idx + 1]).to.eq('2');
+    });
+
+    test('buildArgs preset jobs: 0 takes precedence over fallbackJobs', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build',
+            jobs: 0
+        };
+        const args = buildArgs(preset, undefined, undefined, 8);
+        expect(args).to.include('-j');
+        expect(args).to.not.include('--parallel');
+    });
+
+    test('buildArgs uses fallbackJobs: 0 to pass bare -j', () => {
+        const preset: any = {
+            name: 'test',
+            __binaryDir: '/path/to/build'
+        };
+        const args = buildArgs(preset, undefined, undefined, 0);
+        expect(args).to.include('-j');
+        expect(args).to.not.include('--parallel');
+    });
 });