IO.FS.readFile corrupts the contents of files containing null bytes

[eric-wieser]

Prerequisites

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Description

IO.readFile produces a string that is not contained in the original file.

Context

This came up while trying to read /proc/{pid}/cmdline, which contains null bytes but is valid UTF8.

Steps to Reproduce

1. Run the following, adjusting path to be a temporary file of your choice

#eval do
  let path := "/tmp/lean4-4891"
  IO.FS.writeBinFile path
    ("Hello\x00(this is a very secret message that must never be shared) world".toUTF8)
  let x ← IO.FS.readFile path
  IO.println x

Expected behavior: The secret message is visible

Actual behavior: The output is Hello world. The first 64-byte chunk ends at the the ) character, and is discarded due to the null byte.

Versions

4.10.0

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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[eric-wieser]

Ah, mostly a duplicate of #3546