Merge branch 'installer-symlink-option'

This topic branch fixes the detection whether non-admins can create
symbolic links, and tries to work around the bug that it was misdetected
earlier.

It also fixes some minor issues found while working on the symlink
detection.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

installer/install.iss

@@ -331,6 +331,9 @@ var
     // The options chosen at install time, to be written to /etc/install-options.txt
     ChosenOptions:String;
 
+    // Previous Git for Windows version (if upgrading)
+    PreviousGitForWindowsVersion:String;
+
     // Wizard page and variables for the Path options.
     PathPage:TWizardPage;
     RdbPath:array[GP_BashOnly..GP_CmdTools] of TRadioButton;
@@ -556,42 +559,55 @@ begin
         Result:=-1;
 end;
 
-function IsDowngrade(CurrentVersion,PreviousVersion:String):Boolean;
+function VersionCompare(CurrentVersion,PreviousVersion:String):Integer;
 var
     i,j,Current,Previous:Integer;
 begin
-    Result:=False;
+    Result:=0;
     i:=1;
     j:=1;
     while True do begin
-        if j>Length(PreviousVersion) then
+        if j>Length(PreviousVersion) then begin
+	    Result:=+1;
             Exit;
+	end;
         if i>Length(CurrentVersion) then begin
-            Result:=True;
+            Result:=-1;
             Exit;
         end;
         Previous:=NextNumber(PreviousVersion,j);
-        if Previous<0 then
-            Exit;
         Current:=NextNumber(CurrentVersion,i);
+        if Previous<0 then begin
+            if Current>=0 then
+                Result:=+1;
+            Exit;
+	end;
         if Current<0 then begin
-            Result:=True;
+            Result:=-1;
             Exit;
         end;
-        if Current>Previous then
+        if Current>Previous then begin
+            Result:=+1;
             Exit;
+	end;
         if Current<Previous then begin
-            Result:=True;
+            Result:=-1;
             Exit;
         end;
-        if j>Length(PreviousVersion) then
+        if j>Length(PreviousVersion) then begin
+	    if i<=Length(CurrentVersion) then
+	        Result:=+1;
             Exit;
+	end;
         if i>Length(CurrentVersion) then begin
-            Result:=True;
+            Result:=-1;
             Exit;
         end;
         if CurrentVersion[i]<>PreviousVersion[j] then begin
-            Result:=PreviousVersion[j]='.';
+            if PreviousVersion[j]='.' then
+                Result:=-1
+            else
+                Result:=+1;
             Exit;
         end;
         if CurrentVersion[i]<>'.' then
@@ -612,7 +628,7 @@ end;
 
 function InitializeSetup:Boolean;
 var
-    CurrentVersion,PreviousVersion,Msg:String;
+    CurrentVersion,Msg:String;
     Version:TWindowsVersion;
     ErrorCode:Integer;
 begin
@@ -634,10 +650,11 @@ begin
         Result:=True;
     end;
 #endif
+    RegQueryStringValue(HKEY_LOCAL_MACHINE,'Software\GitForWindows','CurrentVersion',PreviousGitForWindowsVersion);
 #if APP_VERSION!='0-test'
-    if Result and not ParamIsSet('ALLOWDOWNGRADE') and RegQueryStringValue(HKEY_LOCAL_MACHINE,'Software\GitForWindows','CurrentVersion',PreviousVersion) then begin
+    if Result and not ParamIsSet('ALLOWDOWNGRADE') then begin
         CurrentVersion:=ExpandConstant('{#APP_VERSION}');
-        if (IsDowngrade(CurrentVersion,PreviousVersion)) then begin
+        if (VersionCompare(CurrentVersion,PreviousGitForWindowsVersion)<0) then begin
             if WizardSilent() and (ParamIsSet('SKIPDOWNGRADE') or ParamIsSet('VSNOTICE')) then begin
                 Msg:='Skipping downgrade from '+PreviousVersion+' to '+CurrentVersion;
                 if ParamIsSet('SKIPDOWNGRADE') or (ExpandConstant('{log}')='') then
@@ -717,20 +734,26 @@ begin
   ShellExec('','https://github.com/git-for-windows/git/wiki/Symbolic-Links','','',SW_SHOW,ewNoWait,ExitStatus);
 end;
 
+function IsOriginalUserAdmin():Boolean;
+var
+    ResultCode:Integer;
+begin
+    if not ExecAsOriginalUser(ExpandConstant('{cmd}'),ExpandConstant('/c net session >"{tmp}\net-session.txt"'),'',SW_HIDE,ewWaitUntilTerminated,ResultCode) then
+        ResultCode:=-1;
+    Result:=(ResultCode=0);
+end;
+
 function EnableSymlinksByDefault():Boolean;
 var
     ResultCode:Integer;
-    Output:AnsiString;
 begin
-    if IsAdminLoggedOn then begin
-        // The only way to tell whether non-admin users can create symbolic
-	// links is to try using a non-admin user.
-        Result:=False;
-	Exit;
+    if IsOriginalUserAdmin then begin
+        Log('Symbolic link permission detection failed: running as admin');
+	Result:=False;
+    end else begin
+        ExecAsOriginalUser(ExpandConstant('{cmd}'),ExpandConstant('/c mklink /d "{tmp}\symbolic link" "{tmp}" >"{tmp}\symlink test.txt"'),'',SW_HIDE,ewWaitUntilTerminated,ResultCode);
+        Result:=DirExists(ExpandConstant('{tmp}\symbolic link'));
     end;
-
-    ExecAsOriginalUser(ExpandConstant('{cmd}'),ExpandConstant('/c mklink /d "{tmp}\symbolic link" "{tmp}" >"{tmp}\symlink test.txt"'),'',SW_HIDE,ewWaitUntilTerminated,ResultCode);
-    Result:=DirExists(ExpandConstant('{tmp}\symbolic link'));
 end;
 
 function GetTextWidth(Text:String;Font:TFont):Integer;
@@ -1353,7 +1376,7 @@ begin
     // by running `mklink` (unless started as administrator, in which case that
     // test would be meaningless).
     Data:=ReplayChoice('Enable Symlinks','Auto');
-    if Data='Auto' then begin
+    if (Data='Auto') Or ((Data='Disabled') And (VersionCompare(PreviousGitForWindowsVersion,'2.14.1')<=0)) then begin
         if EnableSymlinksByDefault() then
 	    Data:='Enabled'
 	else