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libexpr: improve error reporting if NIX_PATH item cannot be downloaded

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Last week I was hunting a production issue at work related to CppNix and
the only hint I had was this warning. While the cause was clearly a
CppNix issue since it was related to the libgit2-backed tarball cache,
the fact that this exception was hidden, made it way harder to discover
this. It would've saved me a lot of time[1] if the error was just
printed out (and thus would've ended up in the logs).

This is what this patch results in:

  $ nix-instantiate -I https://notexisting.com --find-file . --option connect-timeout 1
  warning: error: unable to download 'https://notexisting.com': Resolving timed out after 1001 milliseconds (curl error code=28); retrying in 265 ms
  warning: error: unable to download 'https://notexisting.com/': Connection timed out after 1002 milliseconds (curl error code=28); retrying in 523 ms
  warning: error: unable to download 'https://notexisting.com/': Connection timed out after 1001 milliseconds (curl error code=28); retrying in 1324 ms
  warning: error: unable to download 'https://notexisting.com/': Connection timed out after 1000 milliseconds (curl error code=28); retrying in 2670 ms
  warning:
         … while downloading https://notexisting.com to satisfy NIX_PATH lookup (ignoring)

         warning: unable to download 'https://notexisting.com/': Connection timed out after 1001 milliseconds (curl error code=28)
  /path/to/nix/path/root

The `unable to download` part was hidden before. Granted, this is not the most
interesting showcase given the retries before, but I'd still argue that
this is useful for cases where you don't have a connection timeout (and
this was just the most trivial way to test for me).

[1] Unlucky as I was, I had to mount an older backup and nixos-enter
    into the mount to reproduce this.

Change-Id: If2894369fc0b159a307b448e1ce7d9b54a327df6
This commit is contained in:
Maximilian Bosch
2025-03-02 23:31:11 +01:00
parent 20fea96996
commit 079fe0ca50
2 changed files with 24 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files
doc/manual/rl-next/improve-nix_path-error.md
+21
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@@ -0,0 +1,21 @@
---
synopsis: Show error when item from NIX_PATH cannot be downloaded
issues: []
cls: []
category: Fixes
credits: [ma27]
---
For e.g. `nix-instantiate -I https://example.com/404`, you'd only get a warning if the download failed, such as
warning: Nix search path entry 'https://example.com/404' cannot be downloaded, ignoring
Now, the full error that caused the download failure is displayed with a note that the search
path entry is ignored, e.g.
warning:
… while downloading https://example.com/404 to satisfy NIX_PATH lookup, ignoring search path entry
warning: unable to download 'https://example.com/404': HTTP error 404 ()
response body: […]
lix/libexpr/eval.cc
+3 -3
View File
@@ -1,6 +1,7 @@
#include "lix/libexpr/eval.hh"
#include "lix/libexpr/eval-settings.hh"
#include "lix/libutil/async.hh"
#include "lix/libutil/error.hh"
#include "lix/libutil/hash.hh"
#include "lix/libexpr/primops.hh"
#include "lix/libexpr/print-options.hh"
@@ -2824,9 +2825,8 @@ try {
store, EvalSettings::resolvePseudoUrl(value), "source", false)).tree.storePath;
res = { store->toRealPath(storePath) };
} catch (FileTransferError & e) {
logWarning({
.msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
});
e.addTrace(nullptr, "while downloading %s to satisfy NIX_PATH lookup, ignoring search path entry", value);
logWarning(e.info());
res = std::nullopt;
}
}