libutil: Add openNewFileForWrite helper function, wrap callsites

This is purely a fix to use CreateFileW in mingw builds. Also adds some
FIXMEs for suspicious symlink following on truncation that can probably
be tightened down without any problems (other than nix-channel), but for
now this is a no-op change other than consistently using O_CLOEXEC, which
is harmless.

src/libexpr/eval-profiler.cc

@@ -133,11 +133,17 @@ class SampleStack : public EvalProfiler
     FrameInfo getPrimOpFrameInfo(const PrimOp & primOp, std::span<Value *> args, PosIdx pos);
 
 public:
-    SampleStack(EvalState & state, std::filesystem::path profileFile, std::chrono::nanoseconds period)
+    SampleStack(EvalState & state, const std::filesystem::path & profileFile, std::chrono::nanoseconds period)
         : state(state)
         , sampleInterval(period)
         , profileFd([&]() {
-            AutoCloseFD fd = toDescriptor(open(profileFile.string().c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0660));
+            AutoCloseFD fd = openNewFileForWrite(
+                profileFile,
+                0660,
+                {
+                    .truncateExisting = true,
+                    .followSymlinksOnTruncate = true, /* FIXME: Probably shouldn't follow symlinks. */
+                });
             if (!fd)
                 throw SysError("opening file %s", PathFmt(profileFile));
             return fd;

src/libstore/build/derivation-building-goal.cc

@@ -1035,14 +1035,13 @@ LogFile::LogFile(Store & store, const StorePath & drvPath, const LogFileSettings
 
     Path logFileName = fmt("%s/%s%s", dir, baseName.substr(2), logSettings.compressLog ? ".bz2" : "");
 
-    fd = toDescriptor(open(
-        logFileName.c_str(),
-        O_CREAT | O_WRONLY | O_TRUNC
-#ifndef _WIN32
-            | O_CLOEXEC
-#endif
-        ,
-        0666));
+    fd = openNewFileForWrite(
+        logFileName,
+        0666,
+        {
+            .truncateExisting = true,
+            .followSymlinksOnTruncate = true, /* FIXME: Probably shouldn't follow symlinks. */
+        });
     if (!fd)
         throw SysError("creating log file '%1%'", logFileName);
 

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

@@ -198,6 +198,29 @@ Descriptor openDirectory(const std::filesystem::path & path);
  */
 Descriptor openFileReadonly(const std::filesystem::path & path);
 
+struct OpenNewFileForWriteParams
+{
+    /**
+     * Whether to truncate an existing file.
+     */
+    bool truncateExisting:1 = false;
+    /**
+     * Whether to follow symlinks if @ref truncateExisting is true.
+     */
+    bool followSymlinksOnTruncate:1 = false;
+};
+
+/**
+ * Open a `Descriptor` for write access or create it if it doesn't exist or truncate existing depending on @ref
+ * truncateExisting.
+ *
+ * @param mode POSIX permission bits. Ignored on Windows.
+ * @throws Nothing.
+ *
+ * @todo Reparse points on Windows.
+ */
+Descriptor openNewFileForWrite(const std::filesystem::path & path, mode_t mode, OpenNewFileForWriteParams params);
+
 /**
  * Read the contents of a file into a string.
  */

src/libutil/unix/file-system.cc

@@ -32,6 +32,19 @@ Descriptor openFileReadonly(const std::filesystem::path & path)
     return open(path.c_str(), O_RDONLY | O_CLOEXEC);
 }
 
+Descriptor openNewFileForWrite(const std::filesystem::path & path, mode_t mode, OpenNewFileForWriteParams params)
+{
+    auto flags = O_WRONLY | O_CREAT | O_CLOEXEC;
+    if (params.truncateExisting) {
+        flags |= O_TRUNC;
+        if (!params.followSymlinksOnTruncate)
+            flags |= O_NOFOLLOW;
+    } else {
+        flags |= O_EXCL; /* O_CREAT | O_EXCL already ensures that symlinks are not followed. */
+    }
+    return open(path.c_str(), flags, mode);
+}
+
 std::filesystem::path defaultTempDir()
 {
     return getEnvNonEmpty("TMPDIR").value_or("/tmp");

src/libutil/windows/file-system.cc

@@ -41,6 +41,19 @@ Descriptor openFileReadonly(const std::filesystem::path & path)
         /*hTemplateFile=*/nullptr);
 }
 
+Descriptor
+openNewFileForWrite(const std::filesystem::path & path, [[maybe_unused]] mode_t mode, OpenNewFileForWriteParams params)
+{
+    return CreateFileW(
+        path.c_str(),
+        GENERIC_WRITE,
+        FILE_SHARE_READ | FILE_SHARE_DELETE,
+        /*lpSecurityAttributes=*/nullptr,
+        params.truncateExisting ? CREATE_ALWAYS : CREATE_NEW, /* TODO: Reparse points. */
+        FILE_ATTRIBUTE_NORMAL,
+        /*hTemplateFile=*/nullptr);
+}
+
 std::filesystem::path defaultTempDir()
 {
     wchar_t buf[MAX_PATH + 1];

src/nix/nix-channel/nix-channel.cc

@@ -44,7 +44,13 @@ static void readChannels()
 // Writes the list of channels.
 static void writeChannels()
 {
-    auto channelsFD = AutoCloseFD{open(channelsList.c_str(), O_WRONLY | O_CLOEXEC | O_CREAT | O_TRUNC, 0644)};
+    AutoCloseFD channelsFD = openNewFileForWrite(
+        channelsList,
+        0644,
+        {
+            .truncateExisting = true,
+            .followSymlinksOnTruncate = true, /* Back-compat. */
+        });
     if (!channelsFD)
         throw SysError("opening '%1%' for writing", channelsList.string());
     for (const auto & channel : channels)

src/nix/prefetch.cc

@@ -112,7 +112,7 @@ std::tuple<StorePath, Hash> prefetchFile(
             if (executable)
                 mode = 0700;
 
-            AutoCloseFD fd = toDescriptor(open(tmpFile.string().c_str(), O_WRONLY | O_CREAT | O_EXCL, mode));
+            AutoCloseFD fd = openNewFileForWrite(tmpFile, mode, {.truncateExisting = false});
             if (!fd)
                 throw SysError("creating temporary file %s", PathFmt(tmpFile));
 
@@ -126,9 +126,9 @@ std::tuple<StorePath, Hash> prefetchFile(
         /* Optionally unpack the file. */
         if (unpack) {
             Activity act(*logger, lvlChatty, actUnknown, fmt("unpacking '%s'", url.to_string()));
-            auto unpacked = (tmpDir.path() / "unpacked").string();
+            auto unpacked = tmpDir.path() / "unpacked";
             createDirs(unpacked);
-            unpackTarfile(tmpFile.string(), unpacked);
+            unpackTarfile(tmpFile, unpacked);
 
             auto entries = DirectoryIterator{unpacked};
             /* If the archive unpacks to a single file/directory, then use