libutil: Add unix::readLinkAt function

This will be used in a following commit.

src/libutil-tests/unix/file-descriptor.cc

@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include <gmock/gmock.h>
 
 #include "nix/util/file-descriptor.hh"
 #include "nix/util/file-system.hh"
@@ -194,4 +195,57 @@ TEST(fchmodatTryNoFollow, fallbackWithoutProc)
 }
 #endif
 
+/* ----------------------------------------------------------------------------
+ * readLinkAt
+ * --------------------------------------------------------------------------*/
+
+TEST(readLinkAt, works)
+{
+    std::filesystem::path tmpDir = nix::createTempDir();
+    nix::AutoDelete delTmpDir(tmpDir, /*recursive=*/true);
+
+    std::string mediumTarget(PATH_MAX / 2, 'x');
+    std::string longTarget(PATH_MAX - 1, 'y');
+
+    {
+        RestoreSink sink(/*startFsync=*/false);
+        sink.dstPath = tmpDir;
+        sink.dirFd = openDirectory(tmpDir);
+        sink.createSymlink(CanonPath("link"), "target");
+        sink.createSymlink(CanonPath("relative"), "../relative/path");
+        sink.createSymlink(CanonPath("absolute"), "/absolute/path");
+        sink.createSymlink(CanonPath("medium"), mediumTarget);
+        sink.createSymlink(CanonPath("long"), longTarget);
+        sink.createDirectory(CanonPath("a"));
+        sink.createDirectory(CanonPath("a/b"));
+        sink.createSymlink(CanonPath("a/b/link"), "nested_target");
+        sink.createRegularFile(CanonPath("regular"), [](CreateRegularFileSink &) {});
+        sink.createDirectory(CanonPath("dir"));
+    }
+
+    AutoCloseFD dirFd = openDirectory(tmpDir);
+
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("link")), "target");
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("relative")), "../relative/path");
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("absolute")), "/absolute/path");
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("medium")), mediumTarget);
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("long")), longTarget);
+    EXPECT_EQ(readLinkAt(dirFd.get(), CanonPath("a/b/link")), "nested_target");
+
+    AutoCloseFD subDirFd = openDirectory(tmpDir / "a");
+    EXPECT_EQ(readLinkAt(subDirFd.get(), CanonPath("b/link")), "nested_target");
+
+    EXPECT_THAT(
+        [&] { readLinkAt(dirFd.get(), CanonPath("regular")); },
+        Throws<SysError>(::testing::Field(&SysError::errNo, EINVAL)));
+
+    EXPECT_THAT(
+        [&] { readLinkAt(dirFd.get(), CanonPath("dir")); },
+        Throws<SysError>(::testing::Field(&SysError::errNo, EINVAL)));
+
+    EXPECT_THAT(
+        [&] { readLinkAt(dirFd.get(), CanonPath("nonexistent")); },
+        Throws<SysError>(::testing::Field(&SysError::errNo, ENOENT)));
+}
+
 } // namespace nix

src/libutil/include/nix/util/file-descriptor.hh

@@ -268,6 +268,11 @@ Descriptor openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPath & p
  */
 void fchmodatTryNoFollow(Descriptor dirFd, const CanonPath & path, mode_t mode);
 
+/*
+ * Read a symlink relative to a directory file descriptor.
+ */
+std::string readLinkAt(Descriptor dirFd, const CanonPath & path);
+
 } // namespace unix
 #endif
 

src/libutil/unix/file-descriptor.cc

@@ -411,4 +411,20 @@ Descriptor unix::openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPa
     return openFileEnsureBeneathNoSymlinksIterative(dirFd, path, flags, mode);
 }
 
+std::string unix::readLinkAt(Descriptor dirFd, const CanonPath & path)
+{
+    assert(!path.isRoot());
+    assert(!path.rel().starts_with('/')); /* Just in case the invariant is somehow broken. */
+    std::vector<char> buf;
+    for (ssize_t bufSize = PATH_MAX / 4; true; bufSize += bufSize / 2) {
+        checkInterrupt();
+        buf.resize(bufSize);
+        ssize_t rlSize = ::readlinkat(dirFd, path.rel_c_str(), buf.data(), bufSize);
+        if (rlSize == -1)
+            throw SysError("reading symbolic link '%1%'", path);
+        else if (rlSize < bufSize)
+            return {buf.data(), static_cast<std::size_t>(rlSize)};
+    }
+}
+
 } // namespace nix