Fix Cocoa 512 errors logged from RKRequestCache. fixes #246

The following changes were made:
* Added isCacheable to RKRequest
* Return nil for cacheKey on non-cacheable RKRequests
* Updated RKRequestCache to ensure attempts to cache uncacheable requests has no effect
* Added basic unit tests and expanded comments on some parts of the cache API

Code/Network/RKRequest.h

@@ -180,11 +180,36 @@ typedef enum RKRequestBackgroundPolicy {
 /// @name Cacheing
 /////////////////////////////////////////////////////////////////////////
 
+/**
+ Returns the cache key for getting/setting the cache entry for this request
+ in the cache.
+ 
+ The cacheKey is an MD5 value computed by hashing a combination of the destination
+ URL, the HTTP verb, and the request body (if possible)
+ */
 @property (nonatomic, readonly) NSString* cacheKey;
 
+/**
+ The cache policy used when storing this request into the request cache
+ */
 @property (nonatomic, assign) RKRequestCachePolicy cachePolicy;
+
+/**
+ The request cache to store and load responses for this request
+ 
+ Generally configured by the RKClient instance that minted this request
+ */
 @property (nonatomic, retain) RKRequestCache* cache;
 
+/**
+ Returns YES if the request is cacheable
+ 
+ All requets are considered cacheable unless:
+    1) The method is DELETE
+    2) The request body is a stream (i.e. using RKParams)
+ */
+- (BOOL)isCacheable;
+
 /**
  * The HTTP body as a NSData used for this request
  */ 

Code/Network/RKRequest.m

@@ -562,20 +562,36 @@
 #endif
 }
 
-- (NSString*)cacheKey {
+- (BOOL)isCacheable {
+    // DELETE is not cacheable
     if (_method == RKRequestMethodDELETE) {
+        return NO;
+    }
+    
+    // Multi-part file uploads are not cacheable
+    if (_params && ![_params respondsToSelector:@selector(HTTPBody)]) {
+        return NO;
+    }
+    
+    return YES;
+}
+
+- (NSString*)cacheKey {
+    if (! [self isCacheable]) {
+        RKLogDebug(@"Asked to return cacheKey for uncacheable request: %@", self);
         return nil;
     }
+    
     // Use [_params HTTPBody] because the URLRequest body may not have been set up yet.
-    NSString* compositCacheKey = nil;
-    if (_params) {
-        if ([_params respondsToSelector:@selector(HTTPBody)]) {
-            compositCacheKey = [NSString stringWithFormat:@"%@-%d-%@", self.URL, _method, [_params HTTPBody]];
-        }
+    NSString* compositeCacheKey = nil;
+    if (_params && [_params respondsToSelector:@selector(HTTPBody)]) {
+        compositeCacheKey = [NSString stringWithFormat:@"%@-%d-%@", self.URL, _method, [_params HTTPBody]];
     } else {
-        compositCacheKey = [NSString stringWithFormat:@"%@-%d", self.URL, _method];
+        compositeCacheKey = [NSString stringWithFormat:@"%@-%d", self.URL, _method];
     }
-    return [compositCacheKey MD5];
+    
+    NSAssert(compositeCacheKey, @"Expected a cacheKey to be generated for request %@, but got nil", compositeCacheKey);
+    return [compositeCacheKey MD5];
 }
 
 @end

Code/Network/RKRequestCache.m

@@ -89,6 +89,10 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (NSString*)pathForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return nil;
+    }
+    
 	[_cacheLock lock];
 
 	NSString* pathForRequest = nil;
@@ -109,6 +113,10 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (BOOL)hasResponseForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return NO;
+    }
+    
 	[_cacheLock lock];
 
 	BOOL hasEntryForRequest = NO;
@@ -138,6 +146,10 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (void)storeResponse:(RKResponse*)response forRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return;
+    }
+    
 	[_cacheLock lock];
     
 	if ([self hasResponseForRequest:request]) {
@@ -186,6 +198,10 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (RKResponse*)responseForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return nil;
+    }
+    
 	[_cacheLock lock];
 
 	RKResponse* response = nil;
@@ -206,6 +222,9 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (NSDictionary*)headersForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return nil;
+    }
     NSString* cachePath = [self pathForRequest:request];
     
     [_cacheLock lock];
@@ -228,6 +247,9 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (NSString*)etagForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return nil;
+    }
 	NSString* etag = nil;
 
     NSDictionary* responseHeaders = [self headersForRequest:request];
@@ -247,6 +269,9 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (void)setCacheDate:(NSDate*)date forRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return;
+    }
     NSMutableDictionary* responseHeaders = [[self headersForRequest:request] mutableCopy];
     
     [responseHeaders setObject:[[RKRequestCache rfc1123DateFormatter] stringFromDate:date]
@@ -257,6 +282,9 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (NSDate*)cacheDateForRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return nil;
+    }
 	NSDate* date = nil;
     NSString* dateString = nil;
     
@@ -278,6 +306,10 @@ static NSDateFormatter* __rfc1123DateFormatter;
 }
 
 - (void)invalidateRequest:(RKRequest*)request {
+    if (! [request isCacheable]) {
+        return;
+    }
+    
 	[_cacheLock lock];
     RKLogDebug(@"Invalidating cache entry for '%@'", request);