Ensure that memory-maped files are fully realized.

base/files/memory_mapped_file_posix.cc

Index: base/files/memory_mapped_file_posix.cc
diff --git a/base/files/memory_mapped_file_posix.cc b/base/files/memory_mapped_file_posix.cc
index 90ba6f49c1573f6fb4fb1f56bc33f35b36febb06..9028757c49a675d5d74c8ca1859de0b4d4af6e4a 100644
--- a/base/files/memory_mapped_file_posix.cc
+++ b/base/files/memory_mapped_file_posix.cc
@@ -4,6 +4,7 @@
 
 #include "base/files/memory_mapped_file.h"
 
+#include <fcntl.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/mman.h>
@@ -14,6 +15,10 @@
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
 
+#if defined(OS_ANDROID)
+#include <android/api-level.h>
+#endif
+
 namespace base {
 
 MemoryMappedFile::MemoryMappedFile() : data_(NULL), length_(0) {
@@ -71,22 +76,72 @@ bool MemoryMappedFile::MapFileRegionToMemory(
     case READ_ONLY:
       flags |= PROT_READ;
       break;
+
     case READ_WRITE:
       flags |= PROT_READ | PROT_WRITE;
       break;
+
     case READ_WRITE_EXTEND:
+      flags |= PROT_READ | PROT_WRITE;
+
       // POSIX won't auto-extend the file when it is written so it must first
       // be explicitly extended to the maximum size. Zeros will fill the new
       // space.
-      auto file_len = file_.GetLength();
-      if (file_len < 0) {
+      const int64_t original_file_len = file_.GetLength();
+      if (original_file_len < 0) {
         DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
         return false;
       }
-      file_.SetLength(std::max(file_len, region.offset + region.size));
-      flags |= PROT_READ | PROT_WRITE;
+
+      // Increase the actual length of the file, if necessary. This can fail if
+      // the disk is full and the OS doesn't support sparse files.
+      if (!file_.SetLength(
+              std::max(original_file_len, region.offset + region.size))) {
+        DPLOG(ERROR) << "ftruncate " << file_.GetPlatformFile();
+        return false;
+      }
+
+      // Realize the extent of the file so that it can't fail (and crash) later
+      // when trying to write to a memory page that can't be created. This can
+      // fail if the disk is full and the file is sparse.
+      //
+      // Only Android API>=21 supports the fallocate call. Older versions need
+      // to manually extend the file by writing zeros at block intervals.
+      //
+      // Mac OSX doesn't support this call but the primary filesystem doesn't
+      // support sparse files so is unneeded.
+      bool do_manual_extension = false;
+
+#if defined(OS_ANDROID) && __ANDROID_API__ < 21
+      do_manual_extension = true;
+#elif !defined(OS_MACOSX)
+      if (posix_fallocate(file_.GetPlatformFile(), region.offset,
+                          region.size) != 0) {
+        DPLOG(ERROR) << "posix_fallocate " << file_.GetPlatformFile();
+        // This can fail because the filesystem doesn't support it so don't
+        // give up just yet. Try the manual method below.
+        do_manual_extension = true;
+      }
+#endif
+
+      // Manually realize the entire file by writing bytes to it at intervals.
+      if (do_manual_extension) {
+        int64_t block_size = 1024;  // Start with something safe.
+        struct stat statbuf;
+        if (fstat(file_.GetPlatformFile(), &statbuf) == 0)
+          block_size = statbuf.st_blksize;
+        const off_t map_end = map_start + static_cast<off_t>(map_size);
+        for (off_t i = map_start; i < map_end; i += block_size) {
+          if (i > original_file_len) {

[Mark Mentovai, 2017/05/05 18:31:44]

I don’t see anything here that requires file_ to have not been a sparse file to
begin with. It’d be safer to get rid of this check and instead probe with pread,
pwrite-ing over any byte that shows up as 0.

[bcwhite, 2017/05/08 14:15:35]

Correct.  It's making the assumption that whatever previously wrote the file
didn't leave any holes.  I was trying to be efficient but I guess it's
read-modify-write of the block any way we look at it so probably better to be
safe.

+            if (pwrite(file_.GetPlatformFile(), "", 1, i) != 1)
+              return false;
+          }
+        }
+      }
+
       break;
   }
+
   data_ = static_cast<uint8_t*>(mmap(NULL, map_size, flags, MAP_SHARED,
                                      file_.GetPlatformFile(), map_start));
   if (data_ == MAP_FAILED) {