Set the minimum Xcode version for the generate docset

Require at least the major version associated with the docsetutil binary
used to generate the docset. This prevents older Xcode versions from
loading the docset and likely crashing as a result.

doctool/DTDocSetGenerator.m

@@ -4,6 +4,7 @@
  */
 
 #import "DTDocSetGenerator.h"
+#import "DocTool.h"
 #import "GRMustache.h"
 
 @implementation DTDocSetGenerator
@@ -15,17 +16,32 @@ - (void) generateDocumentationForLibrary: (DTLibrary *) library error: (NSError
 
     NSString *contentsDirectory = [docSetOutputDirectory stringByAppendingPathComponent: @"Contents"];
 
-    // TODO: Determine the minimum Xcode version based on the version of the version of Xcode containing
-    // docsetutil. There doesn't seem to be a better way, as docsetutil itself has no version and does
-    // not seem to expose what the minimum required Xcode version is. This has to be right because Xcode
-    // will most likely crash if it tries to load a docset in a format it doesn't understand.
+    NSBundle *xcodeBundle = [self selectedXcodeBundle];
+    if (!xcodeBundle) {
+        if (error) {
+            *error = [NSError errorWithDomain: DTErrorDomain code: DTErrorHTMLOutputGeneration userInfo: @{
+                NSLocalizedDescriptionKey: NSLocalizedString(@"Could not locate selected Xcode bundle.", nil)
+            }];
+        }
+        return;
+    }
+
+    NSString *minimumXcodeVersion = [self docSetMinimumXcodeVersionForBundle: xcodeBundle];
+    if (!minimumXcodeVersion) {
+        if (error) {
+            *error = [NSError errorWithDomain: DTErrorDomain code: DTErrorHTMLOutputGeneration userInfo: @{
+                NSLocalizedDescriptionKey: NSLocalizedString(@"Could not obtain minimum docset version from selected Xcode bundle.", nil)
+            }];
+        }
+    }
+
     NSDictionary *infoPlist = @{
         @"CFBundleIdentifier": self.bundleIdentifier,
         @"CFBundleName": self.bundleName,
         @"CFBundleVersion": self.bundleVersion,
         @"DocSetPublisherIdentifier": self.publisherIdentifier,
         @"DocSetPublisherName": self.publisherName,
-        @"DocSetMinimumXcodeVersion": @"5.0"
+        @"DocSetMinimumXcodeVersion": minimumXcodeVersion
     };
     [infoPlist writeToFile: [contentsDirectory stringByAppendingPathComponent: @"Info.plist"] atomically: NO];
 
@@ -51,4 +67,66 @@ - (void) indexDocSetAtPath: (NSString *) path {
     [task waitUntilExit];
 }
 
+/**
+ * Return the minimum Xcode version needed for compatibility with the generted docset.
+ *
+ * It is important that this be as correct as possible because docsets are not backward
+ * compatible and if Xcode loads a docset it does not understand it will likely crash.
+ * Unfortunately docsetutil does not appear to have any way to query the required Xcode
+ * version and this information is not published anywhere. As docsetutil itself does not
+ * have a version number, obtain the major version of the selected Xcode bundle and use
+ * this as the minimum version.
+ */
+- (NSString *) docSetMinimumXcodeVersionForBundle: (NSBundle *) bundle {
+    NSString *xcodeVersion = bundle.infoDictionary[@"CFBundleShortVersionString"];
+    int majorVersion = 0;
+    NSScanner *scanner = [NSScanner scannerWithString: xcodeVersion];
+    if ([scanner scanInt:&majorVersion] == NO)
+        return nil;
+
+    return [NSString stringWithFormat:@"%d.0", majorVersion];
+}
+
+/**
+ * Return the currently selected Xcode bundle, as identified by xcode-select --print-path
+ *
+ * This is used instead of querying Launch Services because Launch Services will return the
+ * bundle with the highest version number, while this method will return the bundle used to
+ * service xcrun invocations.
+ */
+- (NSBundle *) selectedXcodeBundle {
+    NSBundle *result = nil;
+    NSPipe *outputPipe = [NSPipe pipe];
+    NSTask *task = [[NSTask alloc] init];
+    task.launchPath = @"/usr/bin/xcode-select";
+    task.arguments = @[@"--print-path"];
+    task.standardInput = [NSPipe pipe];
+    task.standardOutput = outputPipe;
+    task.standardError = [NSPipe pipe];
+    [task launch];
+    [task waitUntilExit];
+
+    NSData *pathData = [outputPipe.fileHandleForReading readDataToEndOfFile];
+    NSString *path = [[NSString alloc] initWithData: pathData encoding: NSUTF8StringEncoding];
+    path = [path stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
+
+    // Use a URL here because isFilePackageAtPath: is part of AppKit
+    NSURL *fileURL = [NSURL fileURLWithPath: path];
+    NSUInteger count = [[fileURL pathComponents] count];
+    if (count == 0)
+        return result;
+
+    for (NSUInteger i = 0; i < count - 1; i++) {
+        NSNumber *isPackage = nil;
+        [fileURL getResourceValue:&isPackage forKey: NSURLIsPackageKey error: nil];
+        if ([isPackage boolValue]) {
+            result = [NSBundle bundleWithURL:fileURL];
+            break;
+        }
+        fileURL = [fileURL URLByDeletingLastPathComponent];
+    }
+
+    return result;
+}
+
 @end