Work around an apparent bug in libssh2 where it clobbers known_hosts entries...

Work around an apparent bug in libssh2 where it clobbers known_hosts entries with key types it does not recognize. Issue 4250.

sources/NSData+iTerm.h

@@ -25,4 +25,10 @@
 - (BOOL)hasPrefixOfBytes:(char *)bytes length:(int)length;
+
+// Appends this data to the file at |path|. If |addNewline| is YES then a '\n' is appended if the
+// file does not already end with \n or \r. This plays a little fast and loose with character
+// encoding, but it gets the job done.
+- (BOOL)appendToFile:(NSString *)path addLineBreakIfNeeded:(BOOL)addNewline;
+
 @end

sources/NSData+iTerm.m

@@ -7,6 +7,8 @@
 //
 #import "NSData+iTerm.h"
+
+#import "DebugLogging.h"
 #import "RegexKitLite.h"
 #import <apr-1/apr_base64.h>
@@ -105,4 +107,42 @@
     return nil;
 }
+- (BOOL)appendToFile:(NSString *)path addLineBreakIfNeeded:(BOOL)addNewline {
+    NSFileHandle *fileHandle = [NSFileHandle fileHandleForUpdatingAtPath:path];
+    if (!fileHandle) {
+        [[NSFileManager defaultManager] createFileAtPath:path contents:nil attributes:nil];
+        fileHandle = [NSFileHandle fileHandleForWritingAtPath:path];
+        if (!fileHandle) {
+            DLog(@"Failed to open for writing or create %@", path);
+            return NO;
+        }
+    }
+
+    @try {
+        [fileHandle seekToEndOfFile];
+        if (addNewline) {
+            unsigned long long length = fileHandle.offsetInFile;
+            if (length > 0) {
+                [fileHandle seekToFileOffset:length - 1];
+                NSData *data = [fileHandle readDataOfLength:1];
+                if (data.length == 1) {
+                    char lastByte = ((const char *)data.bytes)[0];
+                    if (lastByte != '\r' && lastByte != '\n') {
+                        [fileHandle seekToEndOfFile];
+                        [fileHandle writeData:[NSData dataWithBytes:"\n" length:1]];
+                    }
+                }
+            }
+        }
+        [fileHandle writeData:self];
+        return YES;
+    }
+    @catch (NSException * e) {
+        return NO;
+    }
+    @finally {
+        [fileHandle closeFile];
+    }
+}
+
 @end

sources/SCPFile.m

@@ -13,7 +13,9 @@
 #import <NMSSH/libssh2.h>
 #import "NSFileManager+iTerm.h"
 #import "NSObject+iTerm.h"
-#import "NSStringITerm.h"
+#import "NSData+iTerm.h"
+#import "NSStringiTerm.h"
+#import "NSWorkspace+iTerm.h"
 @interface NMSSHSession(iTerm)
 - (id)agent;
@@ -401,10 +403,21 @@ static NSError *SCPFileError(NSString *description) {
     }
     
     if (_okToAdd) {
+        // Issue 4250 reveals that libssh2 deletes lines out of known_hosts if it
+        // does not recognize them. The workaround is to write to a temp file and then append it to
+        // the real file so libssh2 doesn't have a chance to mess up the real file.
+        NSString *tempFileName = [[NSWorkspace sharedWorkspace] temporaryFileNameWithPrefix:@"iTerm2" suffix:@"knownHost"];
+        [[NSFileManager defaultManager] setAttributes:@{ NSFilePosixPermissions: @(0600) }
+                                         ofItemAtPath:tempFileName
+                                                error:nil];
         [self.session addKnownHostName:self.session.host
                                   port:[self.session.port intValue]
-                                toFile:nil
+                                toFile:tempFileName
                               withSalt:nil];
+        
+        NSData *newEntry = [NSData dataWithContentsOfFile:tempFileName];
+        [[NSFileManager defaultManager] removeItemAtPath:tempFileName error:nil];
+        [newEntry appendToFile:[@"~/.ssh/known_hosts" stringByExpandingTildeInPath] addLineBreakIfNeeded:YES];
     }
     if (isDownload) {