SimpleCache: open with fewer seeks.

net/disk_cache/simple/simple_entry_format.h

Index: net/disk_cache/simple/simple_entry_format.h
diff --git a/net/disk_cache/simple/simple_entry_format.h b/net/disk_cache/simple/simple_entry_format.h
index aec7162f2c2775da3da85e5ed65ad35e5064ff3f..30a369358fb6723aad54a8369a62174cf1dfef7b 100644
--- a/net/disk_cache/simple/simple_entry_format.h
+++ b/net/disk_cache/simple/simple_entry_format.h
@@ -25,7 +25,15 @@ const uint64_t kSimpleSparseRangeMagicNumber = UINT64_C(0xeb97bf016553676b);
 //   - the data from stream 1.
 //   - a SimpleFileEOF record for stream 1.
 //   - the data from stream 0.
+//   - (optionally) the SHA256 of the key.
 //   - a SimpleFileEOF record for stream 0.
+//
+// Because stream 0 data (typically HTTP headers) is on the critical path of
+// requests, on open, the cache reads the end of the record and does not
+// read the SimpleFileHeader. If the key can be validated with a SHA256, then
+// the stream 0 data can be returned to the caller without reading the
+// SimpleFileHeader. If the key SHA256 is not present, then the cache must
+// read the SimpleFileHeader to confirm key equality.
 
 // A file containing stream 2 in the Simple cache consists of:
 //   - a SimpleFileHeader.
@@ -47,6 +55,7 @@ struct NET_EXPORT_PRIVATE SimpleFileHeader {
 struct NET_EXPORT_PRIVATE SimpleFileEOF {
   enum Flags {
     FLAG_HAS_CRC32 = (1U << 0),
+    FLAG_HAS_KEY_SHA256 = (1U << 1),  // Preceding the record if present.
   };
 
   SimpleFileEOF();