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Fix test: hash value depends on endianness and bitness. #10011

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Nov 10, 2021
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Fix test: hash value depends on endianness and bitness. #10011

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b5590ca
Only run test on 64-bit little endian architectures.
hkratz Oct 25, 2021
d4c524e
Reword comment.
hkratz Nov 8, 2021
aa00def
fmt
hkratz Nov 8, 2021
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6 changes: 6 additions & 0 deletions src/cargo/core/source/source_id.rs
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Expand Up @@ -617,7 +617,13 @@ impl Ord for SourceKind {
// you're able to restore the hash to its original value, please do so!
// Otherwise please just leave a comment in your PR as to why the hash value is
// changing and why the old value can't be easily preserved.
//
// The hash value differs depending on endianess and bit-width since Rust 1.45
// (see https://github.com/rust-lang/rust/issues/74215). We run this test only
// on 64-bit little-endian platforms which are most commonly used for
// development and tested in CI.
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I don't think the bit about 1.45 or linking to a Rust issue is really all that relevant here, I think the comment can basically say that the hash value here is machine-specific, so this test is limited to run only on machines where the listed value is actually correct. This platform happens to correspond to x86_64 which is used for CI which means we should see failures.

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The hash-value is machine-specific because of that issue. I think it's not representative of "the whole picture" to say that this test behaviour is "intended for us as developers of Cargo really and not much else." - the hash is used on end-users' filesystems as part of a file path. This is not machine-dependent e.g. when home directories are mounted via NFS and shared across different machines e.g. in a university network.

It may not be a big deal in this particular case, at the very least it will result in some duplication, but I think it's important to understand what the consequences are in the wider context of general computing environments rather than wave it away as a "developer-only" issue.

Stable hashing is a really nice thing to have for protocols and other situations where you want to interoperate, and it's a shame if the Rust standard library wouldn't support it.

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@infinity0 infinity0 Oct 25, 2021 •
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This comment in src/cargo/core/compiler/fingerprint.rs suggests that StableHasher was originally intended to be platform-independent (as would be a reasonable assumption from the naming):

//! Note: Fingerprinting is not a perfect solution. Filesystem mtime tracking
//! is notoriously imprecise and problematic. Only a small part of the
//! environment is captured. This is a balance of performance, simplicity, and
//! completeness. Sandboxing, hashing file contents, tracking every file
//! access, environment variable, and network operation would ensure more
//! reliable and reproducible builds at the cost of being complex, slow, and
//! platform-dependent.

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Ok well in that case I'll leave this to the tagged reviewer.

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@hkratz hkratz Oct 26, 2021 •
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Ok well in that case I'll leave this to the tagged reviewer.

@alexcrichton Oh, I did not see that reply when commenting (below). 😦

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#[test]
#[cfg(all(target_endian = "little", target_pointer_width = "64"))]
fn test_cratesio_hash() {
let config = Config::default().unwrap();
let crates_io = SourceId::crates_io(&config).unwrap();
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