Add function to read pages from RawDiskQueue_TwoFiles

fdbserver/DiskQueue.actor.cpp

@@ -143,6 +143,8 @@ public:
 
 	Future<Void> setPoppedPage( int file, int64_t page, int64_t debugSeq ) { return setPoppedPage(this, file, page, debugSeq); }
 
+	// FIXME: let the caller pass in where to write the data.
+	Future<Standalone<StringRef>> read(int file, int page, int nPages) { return read(this, file, page, nPages); }
 	Future<Standalone<StringRef>> readNextPage() { return readNextPage(this); }
 	Future<Void> truncateBeforeLastReadPage() { return truncateBeforeLastReadPage(this); }
 
@@ -540,6 +542,15 @@ public:
 		}
 	}
 
+	ACTOR static Future<Standalone<StringRef>> read(RawDiskQueue_TwoFiles* self, int file, int pageOffset, int nPages) {
+		state TrackMe trackMe(self);
+		state const size_t bytesRequested = nPages * sizeof(Page);
+		state Standalone<StringRef> result = makeAlignedString(sizeof(Page), bytesRequested);
+		int bytesRead = wait( self->files[file].f->read( mutateString(result), bytesRequested, pageOffset*sizeof(Page) ) );
+		ASSERT_WE_THINK(bytesRead == bytesRequested);
+		return result;
+	}
+
 	Future<int> fillReadingBuffer() {
 		// If we're right at the end of a file...
 		if ( readingPage*sizeof(Page) >= (size_t)files[readingFile].size ) {

flow/Arena.h

@@ -547,6 +547,14 @@ inline static Standalone<StringRef> makeString( int length ) {
 	return returnString;
 }
 
+inline static Standalone<StringRef> makeAlignedString( int alignment, int length ) {
+	Standalone<StringRef> returnString;
+	uint8_t *outData = new (returnString.arena()) uint8_t[alignment + length];
+	outData = (uint8_t*)((((uintptr_t)outData + (alignment - 1)) / alignment) * alignment);
+	((StringRef&)returnString) = StringRef(outData, length);
+	return returnString;
+}
+
 inline static StringRef makeString( int length, Arena& arena ) {
 	uint8_t *outData = new (arena) uint8_t[length];
 	return StringRef(outData, length);