packagefs: Add a fallback buffer for reading heaps in low-memory conditions.

Otherwise we might fail with B_NO_MEMORY, or wait for long periods of
time, which we don't want to do in such a critical routine.

Should improve system responsiveness (or even prevent application
crashes) under high memory pressure.

headers/private/package/hpkg/PackageFileHeapAccessorBase.h

@@ -100,6 +100,7 @@ public:
 	static	const size_t		kChunkSize = 64 * 1024;
 #if defined(_KERNEL_MODE)
 	static	void*				sQuadChunkCache;
+	static	void*				sQuadChunkFallbackBuffer;
 #endif
 
 protected:

src/add-ons/kernel/file_systems/packagefs/kernel_interface.cpp

@@ -1153,7 +1153,8 @@ packagefs_std_ops(int32 op, ...)
 				create_object_cache("pkgfs heap buffers",
 					PackageFileHeapAccessorBase::kChunkSize * 4,
 					0);
-			object_cache_set_minimum_reserve(quadChunkCache, 1);
+			PackageFileHeapAccessorBase::sQuadChunkFallbackBuffer =
+				object_cache_alloc(quadChunkCache, 0);
 
 			TwoKeyAVLTreeNode<void*>::sNodeCache =
 				create_object_cache("pkgfs TKAVLTreeNodes",
@@ -1176,6 +1177,9 @@ packagefs_std_ops(int32 op, ...)
 			PRINT("package_std_ops(): B_MODULE_UNINIT\n");
 			PackageFSRoot::GlobalUninit();
 			delete_object_cache(TwoKeyAVLTreeNode<void*>::sNodeCache);
+			object_cache_free((object_cache*)
+				PackageFileHeapAccessorBase::sQuadChunkCache,
+				PackageFileHeapAccessorBase::sQuadChunkFallbackBuffer, 0);
 			delete_object_cache((object_cache*)
 				PackageFileHeapAccessorBase::sQuadChunkCache);
 			StringConstants::Cleanup();

src/kits/package/hpkg/PackageFileHeapAccessorBase.cpp

@@ -11,7 +11,8 @@
 
 #include <algorithm>
 #include <new>
-#ifdef _KERNEL_MODE
+#if defined(_KERNEL_MODE) && !defined(_BOOT_MODE)
+#include <util/AutoLock.h>
 #include <slab/Slab.h>
 #endif
 
@@ -30,8 +31,10 @@ namespace BHPKG {
 namespace BPrivate {
 
 
-#if defined(_KERNEL_MODE)
+#if defined(_KERNEL_MODE) && !defined(_BOOT_MODE)
 void* PackageFileHeapAccessorBase::sQuadChunkCache = NULL;
+void* PackageFileHeapAccessorBase::sQuadChunkFallbackBuffer = NULL;
+static mutex sFallbackBufferLock = MUTEX_INITIALIZER("PackageFileHeapAccessorBase fallback buffer");
 #endif
 
 
@@ -239,9 +242,16 @@ PackageFileHeapAccessorBase::ReadDataToOutput(off_t offset, size_t size,
 	};
 
 	ObjectCacheDeleter chunkBufferDeleter((object_cache*)sQuadChunkCache);
-	uint8* quadChunkBuffer = (uint8*)object_cache_alloc((object_cache*)sQuadChunkCache, 0);
+	uint8* quadChunkBuffer = (uint8*)object_cache_alloc((object_cache*)sQuadChunkCache,
+		CACHE_DONT_WAIT_FOR_MEMORY);
 	chunkBufferDeleter.object = quadChunkBuffer;
 
+	MutexLocker fallbackBufferLocker(sFallbackBufferLock, false, false);
+	if (quadChunkBuffer == NULL) {
+		fallbackBufferLocker.Lock();
+		quadChunkBuffer = (uint8*)sQuadChunkFallbackBuffer;
+	}
+
 	// segment data buffer
 	iovec localScratch;
 	compressedDataBuffer = (uint16*)(quadChunkBuffer + 0);