hg extdiff -p and %~dp0 - mercurial

Question

myprogram.cmd is in PATH;
myprogram.cmd uses %~dp0 to determine the folder it is in;
I have included #echo %~dp0 into myprogram.cmd for debugging;
When I call myprogram.cmd from anywhere, it works perfectly, displaying the folder myprogram.cmd is in;
When I call hg extdiff -p myprogram.cmd, it does not work, displaying something like c:\Users\Username\AppData\Local\Temp\extdiff.3n8op2\.
Here is the related part of hgrc file:
[extensions]
hgext.extdiff =
What do I do wrong? Should not %~dp0 return drive and path of the batch file? What do I use instead? Do I have to apply some special configuration to Mercurial repository? Passing the full path of myprogram.cmd to hg extdiff -p is not an option, unless it is done automatically.

Answer 1

The %~dp0 trick is a big lie. It is not actually a magic variable, it's simply a manipulation of %0 (or whichever var you stick the ~dp in front of). It simply takes whatever string is in that variable and tells you what its drive and path components appear to be. If that string is simply a name like "myprogram", it says "well, filenames without drives and paths are assumed to be in the current directory".
So the %~dp0 trick only works if either:
a) you've launched your script by its full name or
b) you happen to be in the directory where it resides
In this case, you run:
hg extdiff -p myprogram
gets turned into the following call to Windows:
CreateProcess(NULL, "cmd.exe /c myprogram some diff args", ..., "c:/some/temp/path", ...)
which is morally equivalent to opening a shell and running:
C:\>cd c:\some\temp\path
C:\some\temp\path>myprogram some diff args
%0 is myprogram
%~dp0 is C:\some\temp\path
I recommend passing the full program name for your tool like this via your .hgrc:
[extdiff]
myprogram=c:/tools/myprogram.cmd
But be warned, this can get confused by the presence of spaces in your filename, you may need to experiment with quoting.

Answer 2

You can try using %~dp$PATH:0, but you need to always specify the extension. For example, for the following test.cmd:
rem test.cmd
#echo dp0 == %~dp0
#echo dp$PATH:0 == %~dp$PATH:0
these are two sample runs from the d:\hg folder:
$ hg extdiff -p test.cmd
dp0 == c:\Users\estefan\AppData\Local\Temp\extdiff.pamj6n\
dp$PATH:0 == k:\home\Scripts\
$ hg extdiff -p test
dp0 == c:\Users\estefan\AppData\Local\Temp\extdiff.dgp0qz\
dp$PATH:0 ==
From http://ss64.com/nt/syntax-args.html:
%~$PATH:1 Search the PATH environment variable and expand %1 to the fully qualified name of the first match found.