Obtain a file lock with flock() while creating a separate debug file

common/common.h

@@ -795,6 +795,14 @@ private:
   int perm;
 };
 
+template <typename Context>
+class LockingOutputFile : public OutputFile<Context> {
+public:
+  LockingOutputFile(Context &ctx, std::string path, int perm);
+  void resize(Context &ctx, i64 filesize);
+  void close(Context &ctx) override;
+};
+
 //
 // hyperloglog.cc
 //

common/output-file-unix.h

@@ -2,6 +2,7 @@
 
 #include <fcntl.h>
 #include <filesystem>
+#include <sys/file.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -143,4 +144,52 @@ OutputFile<Context>::open(Context &ctx, std::string path, i64 filesize, int perm
   return std::unique_ptr<OutputFile>(file);
 }
 
+// LockingOutputFile is similar to MemoryMappedOutputFile, but it doesn't
+// rename output files and instead acquires file lock using flock().
+template <typename Context>
+LockingOutputFile<Context>::LockingOutputFile(Context &ctx, std::string path,
+                                              int perm)
+  : OutputFile<Context>(path, 0, true) {
+  this->fd = ::open(path.c_str(), O_RDWR | O_CREAT, perm);
+  if (this->fd == -1)
+    Fatal(ctx) << "cannot open " << path << ": " << errno_string();
+  flock(this->fd, LOCK_EX);
+
+  // We may be overwriting to an existing debug info file. We want to
+  // make the file unusable so that gdb won't use it by accident until
+  // it's ready.
+  u8 buf[256] = {};
+  (void)!!write(this->fd, buf, sizeof(buf));
+}
+
+template <typename Context>
+void LockingOutputFile<Context>::resize(Context &ctx, i64 filesize) {
+  if (ftruncate(this->fd, filesize) == -1)
+    Fatal(ctx) << "ftruncate failed: " << errno_string();
+
+  this->buf = (u8 *)mmap(nullptr, filesize, PROT_READ | PROT_WRITE,
+                         MAP_SHARED, this->fd, 0);
+  if (this->buf == MAP_FAILED)
+    Fatal(ctx) << this->path << ": mmap failed: " << errno_string();
+
+  this->filesize = filesize;
+  mold::output_buffer_start = this->buf;
+  mold::output_buffer_end = this->buf + filesize;
+}
+
+template <typename Context>
+void LockingOutputFile<Context>::close(Context &ctx) {
+  if (!this->is_unmapped)
+    munmap(this->buf, this->filesize);
+
+  if (!this->buf2.empty()) {
+    FILE *out = fdopen(this->fd, "w");
+    fseek(out, 0, SEEK_END);
+    fwrite(&this->buf2[0], this->buf2.size(), 1, out);
+    fclose(out);
+  }
+
+  ::close(this->fd);
+}
+
 } // namespace mold

common/output-file-win32.h

@@ -97,4 +97,17 @@ OutputFile<Context>::open(Context &ctx, std::string path, i64 filesize, int perm
   return std::unique_ptr<OutputFile<Context>>(file);
 }
 
+template <typename Context>
+LockingOutputFile<Context>::LockingOutputFile(Context &ctx, std::string path,
+                                              int perm)
+  : OutputFile<Context>(path, 0, true) {
+  Fatal(ctx) << "LockingOutputFile is not supported on Windows";
+}
+
+template <typename Context>
+void LockingOutputFile<Context>::resize(Context &ctx, i64 filesize) {}
+
+template <typename Context>
+void LockingOutputFile<Context>::close(Context &ctx) {}
+
 } // namespace mold

elf/passes.cc

@@ -1,5 +1,6 @@
 #include "mold.h"
 #include "blake3.h"
+#include "../common/output-file.h"
 
 #include <fstream>
 #include <functional>
@@ -3056,6 +3057,10 @@ template <typename E>
 void write_separate_debug_file(Context<E> &ctx) {
   Timer t(ctx, "write_separate_debug_file");
 
+  // Open an output file early
+  LockingOutputFile<Context<E>> *file =
+    new LockingOutputFile<Context<E>>(ctx, ctx.arg.separate_debug_file, 0666);
+
   // We want to write to the debug info file in background so that the
   // user doesn't have to wait for it to complete.
   if (ctx.arg.detach)
@@ -3087,11 +3092,9 @@ void write_separate_debug_file(Context<E> &ctx) {
 
   // Write to the debug info file as if it were a regular output file.
   compute_section_headers(ctx);
-  i64 filesize = set_osec_offsets(ctx);
+  file->resize(ctx, set_osec_offsets(ctx));
 
-  ctx.output_file =
-    OutputFile<Context<E>>::open(ctx, ctx.arg.separate_debug_file,
-                                 filesize, 0666);
+  ctx.output_file.reset(file);
   ctx.buf = ctx.output_file->buf;
 
   copy_chunks(ctx);
@@ -3102,7 +3105,7 @@ void write_separate_debug_file(Context<E> &ctx) {
   // Reverse-compute a CRC32 value so that the CRC32 checksum embedded to
   // the .gnu_debuglink section in the main executable matches with the
   // debug info file's CRC32 checksum.
-  u32 crc = compute_crc32(0, ctx.buf, filesize);
+  u32 crc = compute_crc32(0, ctx.buf, ctx.output_file->filesize);
 
   std::vector<u8> &buf2 = ctx.output_file->buf2;
   if (!buf2.empty())

test/elf/separate-debug-file.sh

@@ -16,11 +16,12 @@ $CC -c -o $t/a.o $t/a.c -g
 $CC -B. -o $t/exe1 $t/a.o -Wl,--separate-debug-file
 readelf -SW $t/exe1 | grep -Fq .gnu_debuglink
 
+flock $t/exe1 true
+gdb $t/exe1 -ex 'list main' -ex 'quit' | grep -Fq printf
+
 $CC -c -o $t/a.o $t/a.c -g
 $CC -B. -o $t/exe2 $t/a.o -Wl,--separate-debug-file -Wl,--no-build-id
 readelf -SW $t/exe2 | grep -Fq .gnu_debuglink
 
-sleep 1
-
-gdb $t/exe1 -ex 'list main' -ex 'quit' | grep -Fq printf
+flock $t/exe2 true
 gdb $t/exe2 -ex 'list main' -ex 'quit' | grep -Fq printf