Fix race condition in MakeDirs (bazelbuild#644)

If there are 2 concurrent processes creating the same directories, then it will fail

I noticed this on the CI errors of bazelbuild#567, and a co-worker was hitting it
today. I suspect invalidating the worker or other updates triggered big
rebuilds.

I don't know of a perfect repro but, at a 10% rate: invoking Bazel to
build 2 swift libraries in the same BUILD file.

tools/common/file_system.cc

@@ -18,7 +18,9 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#include <iostream>
 #include <string>
+#include <string.h>
 
 #ifdef __APPLE__
 #include <copyfile.h>
@@ -99,7 +101,13 @@ bool MakeDirs(const std::string &path, int mode) {
   struct stat dir_stats;
   if (stat(path.c_str(), &dir_stats) == 0) {
     // Return true if the directory already exists.
-    return S_ISDIR(dir_stats.st_mode);
+    if (!S_ISDIR(dir_stats.st_mode)) {
+      std::cerr << "error: path exists and isn't a directory "
+                << strerror(errno) << " (" << path.c_str() << ") \n";
+      return false;
+    } else {
+      return true;
+    }
   }
 
   // Recurse to create the parent directory.
@@ -108,5 +116,29 @@ bool MakeDirs(const std::string &path, int mode) {
   }
 
   // Create the directory that was requested.
-  return mkdir(path.c_str(), mode) == 0;
+  int mkdir_ret = mkdir(path.c_str(), mode);
+  if (mkdir_ret == 0) {
+    return true;
+  }
+
+  // Save the mkdir errno for better reporting.
+  int mkdir_errno = errno;
+
+  // Deal with a race condition when 2 `MakeDirs` are running at the same time:
+  // one `mkdir` invocation will fail in each recursive call at different
+  // points. Don't recurse here to avoid an infinite loop in failure cases.
+  if (errno == EEXIST && stat(path.c_str(), &dir_stats) == 0) {
+    if (!S_ISDIR(dir_stats.st_mode)) {
+      std::cerr << "error: path exists and isn't a directory "
+                << strerror(errno) << " (" << path.c_str() << ") \n";
+      return false;
+    } else {
+      // Return true if the directory already exists.
+      return true;
+    }
+  }
+
+  std::cerr << "error:" << strerror(mkdir_errno) << " (" << path.c_str()
+            << ") \n";
+  return false;
 }