feat:Add ~/.gemini/system.md fallback for GEMINI_SYSTEM_MD=1|true #15548
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Summary of ChangesHello @facelezzzz, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed! This pull request introduces a significant improvement to how the Highlights
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Code Review
This pull request introduces a fallback mechanism to use a global system prompt from ~/.gemini/system.md when the project-specific one is not found. The implementation is logically correct and includes corresponding documentation updates and tests. However, the core logic relies on fs.existsSync() before reading the file, which introduces a Time-of-Check to Time-of-Use (TOCTOU) race condition. This could lead to an unhandled exception if the file is removed between the check and the read operation. I've provided a high-severity comment with a recommendation to refactor the logic to avoid this anti-pattern by attempting to read the files directly and handling any 'file not found' errors.
| if (!fs.existsSync(systemMdPath)) { | ||
| throw new Error(`missing system prompt file '${systemMdPath}'`); | ||
| if (!systemMdResolution.isSwitch) { | ||
| throw new Error(`missing system prompt file '${systemMdPath}'`); | ||
| } | ||
| const globalSystemMdPath = getGlobalSystemMdPath(); | ||
| if (!globalSystemMdPath) { | ||
| throw new Error( | ||
| `missing system prompt file '${systemMdPath}' (failed to resolve home directory)`, | ||
| ); | ||
| } | ||
| if (!fs.existsSync(globalSystemMdPath)) { | ||
| throw new Error( | ||
| `missing system prompt file '${systemMdPath}' (also checked '${globalSystemMdPath}')`, | ||
| ); | ||
| } | ||
| systemMdPath = globalSystemMdPath; | ||
| } |
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The use of fs.existsSync() followed by a later fs.readFileSync() introduces a Time-of-Check to Time-of-Use (TOCTOU) race condition. If the file is deleted between these two calls, the application will crash with an unhandled error. This is a known anti-pattern in Node.js.
It's more robust to attempt to read the file directly and handle the ENOENT (file not found) error. This avoids the race condition entirely.
I recommend refactoring this logic to attempt reading the files directly instead of checking for existence first. Here's a conceptual example of how this could be structured:
function tryReadFile(path: string): string | null {
try {
return fs.readFileSync(path, 'utf8');
} catch (error) {
if (error.code === 'ENOENT') {
return null; // File not found, which is an expected case here.
}
throw error; // For other errors (e.g., permissions), re-throw.
}
}
// Inside getCoreSystemPrompt...
let systemPromptContent: string | null = null;
if (systemMdResolution.isSwitch) {
systemPromptContent = tryReadFile(systemMdPath);
if (systemPromptContent === null) {
const globalPath = getGlobalSystemMdPath();
if (globalPath) {
systemPromptContent = tryReadFile(globalPath);
}
}
if (systemPromptContent === null) {
// throw error that neither file was found
}
} else {
systemPromptContent = tryReadFile(systemMdPath);
if (systemPromptContent === null) {
// throw error that custom file was not found
}
}
// Then use systemPromptContent to set basePromptThis approach consolidates file reading and error handling, making the code safer and more reliable.
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This issue is not caused by the current commit and was already present in earlier versions.
facelezzzz
changed the title
1 participant
Summary
This PR allows setting GEMINI_SYSTEM_MD=1 or true so that when .gemini/system.md does not exist in the project root, the system will fall back to $HOME/.gemini/system.md.
Details
Although GEMINI_SYSTEM_MD can be used to specify an explicit path, once a path is set, .gemini/system.md inside certain repositories will no longer take effect, which is inconvenient for our team.
What we want instead is:
- Use .gemini/system.md if it exists in the project
- Otherwise, fall back to $HOME/.gemini/system.md
-
This allows our team to keep a consistent global setup while still easily enabling repository-specific .gemini/system.md files when needed.
The behavior is intuitive and provides good flexibility.
Related Issues
#15549
How to Validate
Run test at
packages/core/src/core/prompts.test.tsUse .gemini/system.md in the project directory.
Use ~/.gemini/system.md
Prefer .gemini/system.md in the project directory even when ~/.gemini/system.md exists.
Pre-Merge Checklist