[sql] Validate database files before enabling memory-mapping.

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index 4e3ce261d8e37f7a0044fe7d0445b50e5207ef08..fad4fe30401ed9441193ab2fce31b7f89382dddf 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -204,6 +204,19 @@ class SQL_EXPORT Connection : public base::trace_event::MemoryDumpProvider {
     EVENT_COMMIT,
     EVENT_ROLLBACK,
 
+    // Track success and failure in MmapHelper().  MmapHelper() should record at
+    // most one of these per run.  The case of mapping everything is not
+    // recorded.
+    EVENT_MMAP_META_MISSING,         // No meta table present.
+    EVENT_MMAP_META_FAILURE_READ,    // Failed reading meta table.
+    EVENT_MMAP_META_FAILURE_UPDATE,  // Failed updating meta table.
+    EVENT_MMAP_VFS_FAILURE,          // Failed to access VFS.
+    EVENT_MMAP_FAILED,               // Failure from past run.
+    EVENT_MMAP_FAILED_NEW,           // Read error in this run.
+    EVENT_MMAP_SUCCESS_NEW,          // Read to EOF in this run.
+    EVENT_MMAP_SUCCESS_PARTIAL,      // Read but did not reach EOF.
+    EVENT_MMAP_SUCCESS_NO_PROGRESS,  // Read quota exhausted.
+
     // Leave this at the end.
     // TODO(shess): |EVENT_MAX| causes compile fail on Windows.
     EVENT_MAX_VALUE
@@ -689,6 +702,16 @@ class SQL_EXPORT Connection : public base::trace_event::MemoryDumpProvider {
   // Helper to collect diagnostic info for errors.
   std::string CollectErrorInfo(int error, Statement* stmt) const;
 
+  // Calculates a value appropriate to pass to "PRAGMA mmap_size = ".  So errors
+  // can make it unsafe to map a file, so the file is read using regular I/O,
+  // with any errors causing 0 (don't map anything) to be returned.  If the
+  // entire file is read without error, a large value is returned which will
+  // allow the entire file to be mapped in most cases.
+  //
+  // Results are recorded in the database's meta table for future reference, so
+  // the file should only be read through once.
+  size_t MmapHelper();

[rmcilroy, 2015/11/05 15:34:46]

I'd call this something specific to it's return value - i.e.,
GetAppropriateMmapSize or similar

[Scott Hess - ex-Googler, 2015/11/05 18:25:07]

Acknowledged.

+
   // The actual sqlite database. Will be NULL before Init has been called or if
   // Init resulted in an error.
   sqlite3* db_;