feat: make the siso remote job count configurable

README.md

@@ -764,6 +764,7 @@ See [`example-configs/`](./example-configs/) for annotated templates (`evm.base.
 | `rbeTlsClientCert`        | string (optional)              | TLS client certificate path, replaces the credential helper (`siso` only)        |
 | `rbeTlsClientKey`         | string (optional)              | Private key path for `rbeTlsClientCert` (`siso` only)                            |
 | `rbePlatformProperties`   | object (optional)              | Remote platform properties, replaces the default backend config (`siso` only)    |
+| `rbeRemoteJobs`           | integer (default: 200)         | Maximum number of build steps to run remotely at once (`siso` only)              |
 | `preserveSDK`             | integer (default: 5)           | Number of recent macOS SDKs to keep on disk                                      |
 | `configValidationLevel`   | `strict` \| `warn` \| `none`   | How strictly to validate the config file (default: `strict`)                     |
 | `extends`                 | string (optional)              | Name of a base config to inherit from; arrays concatenate and objects deep-merge |
@@ -818,6 +819,8 @@ gen:
 - Some backends reject outputs outside the working directory. Add
   `clang_diagnostic_dir = "clang-crashreports"` to `gen.args` to keep the clang crash directory in
   the output directory.
+- Set `rbeRemoteJobs` to match the capacity of the backend. For a single build, pass the flag
+  instead: `e build -remote_jobs 500`.
 
 ## Environment variables
 

evm-config.schema.json

@@ -68,6 +68,13 @@
       },
       "description": "Remote platform properties, replacing the default backend config. siso only"
     },
+    "rbeRemoteJobs": {
+      "type": "integer",
+      "minimum": 1,
+      "maximum": 9007199254740991,
+      "description": "Maximum number of build steps to run remotely at once. siso only",
+      "default": 200
+    },
     "root": {
       "type": "string",
       "minLength": 1,

src/e-build.ts

@@ -95,7 +95,7 @@ async function runNinja(
     }
 
     if (config.remoteBuild === 'siso') {
-      ninjaArgs.push(...siso.flags(config, hasExecute).map(String));
+      ninjaArgs.push(...siso.flags(config, hasExecute, ninjaArgs).map(String));
     }
   } else {
     console.info(`${color.info} Building ${target} with remote execution disabled`);

src/types.ts

@@ -79,6 +79,13 @@ const baseConfigShape = z.strictObject({
     .record(z.string(), z.string())
     .describe('Remote platform properties, replacing the default backend config. siso only')
     .optional(),
+  rbeRemoteJobs: z
+    .number()
+    .int()
+    .min(1)
+    .describe('Maximum number of build steps to run remotely at once. siso only')
+    .meta({ default: 200 })
+    .optional(),
   root: z
     .string()
     .min(1)

src/utils/siso.ts

@@ -10,6 +10,8 @@ import type { SanitizedConfig } from '../types.js';
 // scoped to); everything else uses the shared CI/local namespace.
 const DEFAULT_REAPI_INSTANCE = 'projects/electron-rbe/instances/default_instance';
 
+const DEFAULT_REMOTE_JOBS = 200;
+
 type ConfigLike = Pick<
   SanitizedConfig,
   | 'remoteBuild'
@@ -19,6 +21,7 @@ type ConfigLike = Pick<
   | 'rbeTlsClientCert'
   | 'rbeTlsClientKey'
   | 'rbePlatformProperties'
+  | 'rbeRemoteJobs'
   | 'root'
 >;
 
@@ -53,20 +56,30 @@ function getStarFile(envVar: string, filename: string): string {
   return path.resolve(import.meta.dirname, '../../tools', filename);
 }
 
-export function flags(config: ConfigLike, hasExecute: boolean): (string | number)[] {
+export function flags(
+  config: ConfigLike,
+  hasExecute: boolean,
+  userArgs: string[] = [],
+): (string | number)[] {
   if (config.remoteBuild !== 'siso') return [];
 
-  const result: (string | number)[] = [
-    '-remote_jobs',
-    200,
+  const result: (string | number)[] = [];
+
+  // These flags go after the user's and siso keeps the last value it sees, so
+  // leave -remote_jobs out when the user passed their own.
+  if (!userArgs.some((arg) => /^--?remote_jobs(=|$)/.test(arg.trim()))) {
+    result.push('-remote_jobs', config.rbeRemoteJobs ?? DEFAULT_REMOTE_JOBS);
+  }
+
+  result.push(
     // Keep each value in its flag's token: the Windows shell join drops empty arguments.
     `-project=${project(config)}`,
     `-reapi_instance=${reapiInstance(config)}`,
     '-reapi_address',
     reclient.serviceAddress(config),
     '-load',
     getStarFile('ELECTRON_BUILD_TOOLS_MAIN_STAR', 'main.star'),
-  ];
+  );
 
   if (reclient.usesTlsClientAuth(config)) {
     result.push(

tests/siso.spec.mjs

@@ -68,6 +68,33 @@ describe('siso alternative RBE backends', () => {
   });
 });
 
+describe('siso remote jobs', () => {
+  const remoteJobs = (result) => result[result.indexOf('-remote_jobs') + 1];
+
+  it('defaults to 200', () => {
+    expect(remoteJobs(flags({ remoteBuild: 'siso' }, true))).toBe(200);
+  });
+
+  it('uses rbeRemoteJobs when configured', () => {
+    expect(remoteJobs(flags({ remoteBuild: 'siso', rbeRemoteJobs: 500 }, true))).toBe(500);
+  });
+
+  it.each([
+    [['-remote_jobs', '50']],
+    [['-remote_jobs=50']],
+    [['--remote_jobs', '50']],
+    [['--remote_jobs=50']],
+  ])('leaves the flag out when the user passes %j', (userArgs) => {
+    const result = flags({ remoteBuild: 'siso', rbeRemoteJobs: 500 }, true, userArgs);
+    expect(result).not.toContain('-remote_jobs');
+  });
+
+  it('is not fooled by other flags', () => {
+    const result = flags({ remoteBuild: 'siso' }, true, ['-local_jobs', '8', '-remote_jobs_foo=1']);
+    expect(remoteJobs(result)).toBe(200);
+  });
+});
+
 describe('ensureBackendStarlark', () => {
   let root;
   const backendStar = () => path.join(root, 'src/build/config/siso/backend_config/backend.star');