fix: Handle symlinks and FIFOs in nix hash where possible

Fixes https://github.com/NixOS/nix/issues/11756
Fixes https://github.com/NixOS/nix/issues/11681

src/libutil/posix-source-accessor.hh

@@ -43,7 +43,7 @@ struct PosixSourceAccessor : virtual SourceAccessor
     std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override;
 
     /**
-     * Create a `PosixSourceAccessor` and `CanonPath` corresponding to
+     * Create a `PosixSourceAccessor` and `SourcePath` corresponding to
      * some native path.
      *
      * The `PosixSourceAccessor` is rooted as far up the tree as

src/nix/hash.cc

@@ -87,18 +87,35 @@ struct CmdHashBase : Command
                     return std::make_unique<HashSink>(hashAlgo);
             };
 
-            auto path2 = PosixSourceAccessor::createAtRoot(path);
+            auto makeSourcePath = [&]() -> SourcePath {
+                return PosixSourceAccessor::createAtRoot(makeParentCanonical(path));
+            };
+
             Hash h { HashAlgorithm::SHA256 }; // throwaway def to appease C++
             switch (mode) {
             case FileIngestionMethod::Flat:
+            {
+                // While usually we could use the some code as for NixArchive,
+                // the Flat method needs to support FIFOs, such as those
+                // produced by bash process substitution, e.g.:
+                //     nix hash --mode flat <(echo hi)
+                // Also symlinks semantics are unambiguous in the flat case,
+                // so we don't need to go low-level, or reject symlink `path`s.
+                auto hashSink = makeSink();
+                readFile(path, *hashSink);
+                h = hashSink->finish().first;
+                break;
+            }
             case FileIngestionMethod::NixArchive:
             {
+                auto sourcePath = makeSourcePath();
                 auto hashSink = makeSink();
-                dumpPath(path2, *hashSink, (FileSerialisationMethod) mode);
+                dumpPath(sourcePath, *hashSink, (FileSerialisationMethod) mode);
                 h = hashSink->finish().first;
                 break;
             }
             case FileIngestionMethod::Git: {
+                auto sourcePath = makeSourcePath();
                 std::function<git::DumpHook> hook;
                 hook = [&](const SourcePath & path) -> git::TreeEntry {
                     auto hashSink = makeSink();
@@ -109,7 +126,7 @@ struct CmdHashBase : Command
                         .hash = hash,
                     };
                 };
-                h = hook(path2).hash;
+                h = hook(sourcePath).hash;
                 break;
             }
             }

tests/functional/hash-path.sh

@@ -92,3 +92,27 @@ try2 md5 "20f3ffe011d4cfa7d72bfabef7882836"
 rm "$TEST_ROOT/hash-path/hello"
 ln -s x "$TEST_ROOT/hash-path/hello"
 try2 md5 "f78b733a68f5edbdf9413899339eaa4a"
+
+# Flat mode supports process substitution
+h=$(nix hash path --mode flat --type sha256 --base32 <(printf "SMASH THE STATE"))
+[[ 0d9n3r2i4m1zgy0wpqbsyabsfzgs952066bfp8gwvcg4mkr4r5g8 == "$h" ]]
+
+# Flat mode supports process substitution (hash file)
+h=$(nix hash file --type sha256 --base32 <(printf "SMASH THE STATE"))
+[[ 0d9n3r2i4m1zgy0wpqbsyabsfzgs952066bfp8gwvcg4mkr4r5g8 == "$h" ]]
+
+# Symlinks in the ancestry are ok and don't affect the result
+mkdir -p "$TEST_ROOT/simple" "$TEST_ROOT/try/to/mess/with/it"
+echo hi > "$TEST_ROOT/simple/hi"
+ln -s "$TEST_ROOT/simple" "$TEST_ROOT/try/to/mess/with/it/simple-link"
+h=$(nix hash path --type sha256 --base32 "$TEST_ROOT/simple/hi")
+[[ 1xmr8jicvzszfzpz46g37mlpvbzjl2wpwvl2b05psipssyp1sm8h == "$h" ]]
+h=$(nix hash path --type sha256 --base32 "$TEST_ROOT/try/to/mess/with/it/simple-link/hi")
+[[ 1xmr8jicvzszfzpz46g37mlpvbzjl2wpwvl2b05psipssyp1sm8h == "$h" ]]
+
+# nix hash --mode nar does not canonicalize a symlink argument.
+#   Otherwise it can't generate a NAR whose root is a symlink.
+#   If you want to follow the symlink, pass $(realpath -s ...) instead.
+ln -s /non-existent-48cujwe8ndf4as0bne "$TEST_ROOT/symlink-to-nowhere"
+h=$(nix hash path --mode nar --type sha256 --base32 "$TEST_ROOT/symlink-to-nowhere")
+[[ 1bl5ry3x1fcbwgr5c2x50bn572iixh4j1p6ax5isxly2ddgn8pbp == "$h" ]]  # manually verified hash