Support read/write memory-mapped files.

base/files/memory_mapped_file.cc

Index: base/files/memory_mapped_file.cc
diff --git a/base/files/memory_mapped_file.cc b/base/files/memory_mapped_file.cc
index 0fd9d6796b10734a1224556ad16ab0d2418662f9..67890d68855cb810a086da788d2f046f78240d25 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -30,18 +30,30 @@ MemoryMappedFile::~MemoryMappedFile() {
 }
 
 #if !defined(OS_NACL)
-bool MemoryMappedFile::Initialize(const FilePath& file_name) {
+bool MemoryMappedFile::Initialize(const FilePath& file_name, Access access) {
   if (IsValid())
     return false;
 
-  file_.Initialize(file_name, File::FLAG_OPEN | File::FLAG_READ);
+  uint32_t flags = 0;
+  switch (access) {
+    case READ_ONLY:
+      flags = File::FLAG_OPEN | File::FLAG_READ;
+      break;
+    case READ_WRITE:
+      flags = File::FLAG_OPEN | File::FLAG_READ | File::FLAG_WRITE;
+      break;
+    case READ_WRITE_EXTEND:
+      // Can't open with "extend" because no maximum size is known.
+      NOTREACHED();

[erikchen, 2016/05/10 02:21:51]

Should this be a CHECK(false) instead?

[danakj, 2016/05/10 20:52:02]

Only if you think we're going to ship code that does this.

[bcwhite, 2016/05/10 21:00:54]

Not really necessary.  A non-DCHECK build would get flags=0 and fail in
Initialize() below.

+  }
+  file_.Initialize(file_name, flags);
 
   if (!file_.IsValid()) {
     DLOG(ERROR) << "Couldn't open " << file_name.AsUTF8Unsafe();
     return false;
   }
 
-  if (!MapFileRegionToMemory(Region::kWholeFile)) {
+  if (!MapFileRegionToMemory(Region::kWholeFile, access)) {
     CloseHandles();
     return false;
   }
@@ -49,11 +61,32 @@ bool MemoryMappedFile::Initialize(const FilePath& file_name) {
   return true;
 }
 
-bool MemoryMappedFile::Initialize(File file) {
-  return Initialize(std::move(file), Region::kWholeFile);
+bool MemoryMappedFile::Initialize(File file, Access access) {
+  DCHECK_NE(READ_WRITE_EXTEND, access);
+  return Initialize(std::move(file), Region::kWholeFile, access);
 }
 
-bool MemoryMappedFile::Initialize(File file, const Region& region) {
+bool MemoryMappedFile::Initialize(File file,
+                                  const Region& region,
+                                  Access access) {
+  switch (access) {
+    case READ_WRITE_EXTEND:
+      // Ensure that the extended size is within limits of File.
+      if (region.size > std::numeric_limits<int64_t>::max() - region.offset) {
+        DLOG(ERROR) << "Region bounds exceed maximum for base::File.";
+        return false;
+      }
+      // Fall through.
+    case READ_ONLY:
+    case READ_WRITE:
+      // Ensure that the region values are valid.
+      if (region.offset < 0 || region.size < 0) {
+        DLOG(ERROR) << "Region bounds are not valid.";
+        return false;
+      }
+      break;
+  }
+
   if (IsValid())
     return false;
 
@@ -64,7 +97,7 @@ bool MemoryMappedFile::Initialize(File file, const Region& region) {
 
   file_ = std::move(file);
 
-  if (!MapFileRegionToMemory(region)) {
+  if (!MapFileRegionToMemory(region, access)) {
     CloseHandles();
     return false;
   }