facebook/flow · error

content_of_file_input_unsafe: failed to read file

Error message

content_of_file_input_unsafe: failed to read file

What it means

FileInput::content_of_file_input_unsafe reads the file named by the FileName variant with std::fs::read_to_string and expects success (rust_port/crates/flow_server_utils/src/file_input.rs:31-37). It panics when the path does not exist, permissions block the read, or the bytes are not valid UTF-8. The crate deliberately ships the safe twin content_of_file_input() returning Result<String, String>; calling the _unsafe variant is opting into a process-killing panic on ordinary I/O failure.

Source

Thrown at rust_port/crates/flow_server_utils/src/file_input.rs:36

        match self {
            FileInput::FileName(f) => Some(f),
            FileInput::FileContent(Some(f), _) => Some(f),
            _ => None,
        }
    }

    pub fn filename_of_file_input(&self) -> &str {
        match self {
            FileInput::FileName(f) => f,
            FileInput::FileContent(Some(f), _) => f,
            FileInput::FileContent(None, _) => "-",
        }
    }

    pub fn content_of_file_input_unsafe(&self) -> String {
        match self {
            FileInput::FileName(f) => std::fs::read_to_string(f)
                .expect("content_of_file_input_unsafe: failed to read file"),
            FileInput::FileContent(_, content) => content.to_string(),
        }
    }

    pub fn content_of_file_input(&self) -> Result<String, String> {
        match self {
            FileInput::FileName(f) => std::fs::read_to_string(f).map_err(|e| format!("{}", e)),
            FileInput::FileContent(_, content) => Ok(content.to_string()),
        }
    }

    pub fn content_of_file_input_arc(&self) -> Result<Arc<str>, String> {
        match self {
            FileInput::FileName(f) => std::fs::read_to_string(f)
                .map(Arc::<str>::from)
                .map_err(|e| format!("{}", e)),
            FileInput::FileContent(_, content) => Ok(content.clone()),
        }

View on GitHub (pinned to f88ac94bcf)

Solutions

1. Switch to content_of_file_input() (or content_of_file_input_arc()) and handle Err(String) as a normal error
2. Verify the path exists and is readable before constructing FileInput::FileName; prefer FileInput::FileContent to inline contents
3. If non-UTF-8 input is legitimate, read the bytes and transcode explicitly rather than relying on read_to_string
4. Grep for remaining content_of_file_input_unsafe call sites and remove them

Example fix

// before
let content = input.content_of_file_input_unsafe();

// after
let content = input
    .content_of_file_input()
    .map_err(|e| format!("failed to read {}: {e}", input.filename_of_file_input()))?;

// Narrow the enum before reading: only FileName can fail
if matches!(input, FileInput::FileName(_)) {
    let meta = std::fs::metadata(input.filename_of_file_input());
    if meta.is_err() || !meta.unwrap().is_file() {
        return Err(format!("input file missing: {}", input.filename_of_file_input()));
    }
}

fn safe_content(input: &FileInput) -> Result<String, String> {
    match input {
        FileInput::FileName(f) => std::fs::read_to_string(f).map_err(|e| e.to_string()),
        FileInput::FileContent(_, c) => Ok(c.to_string()),
    }
}

When it happens

Trigger: Calling content_of_file_input_unsafe() on FileInput::FileName(p) where p is missing, unreadable, or non-UTF-8 (binary or legacy-encoded files). FileContent variants return the inlined string and can never panic here.

Common situations: Server code paths that took the shortcut variant; stale paths after file deletion or rename; path mismatches across container bind mounts or operating systems; files in non-UTF-8 encodings.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

• check-contents input should be readable
• Unknown exception reading from the server: {}
• Error sending command to server: {}
• invalid line
• invalid column

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/d83373be42bb15b0. Report an issue: GitHub.