build-support: Use mktemp -d in nix-prefetch-*.

Instead of relying on $$ to not collide with an existing path.

Quoting the Bash manual about $$:

> Expands to the process ID of the shell. In a () subshell, it expands
> to the process ID of the current shell, not the subshell.

So, this is different from $BASHPID:

> Expands to the process ID of the current bash process. This differs
> from $$ under certain circumstances, such as subshells that do not
> require bash to be re-initialized.

But even $BASHPID is prone to race conditions if the process IDs wrap
around, so to be on the safe side, we're using mktemp here.

Closes #3784.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/fetchbzr/nix-prefetch-bzr

@@ -43,11 +43,10 @@ fi
 # If we don't know the hash or a path with that hash doesn't exist,
 # download the file and add it to the store.
 if test -z "$finalPath"; then
-    tmpPath=/tmp/bzr-checkout-tmp-$$
-    tmpFile=$tmpPath/$dstFile
-    mkdir $tmpPath
+    tmpPath="$(mktemp --tmpdir -d bzr-checkout-tmp-XXXXXXXX)"
+    trap "rm -rf \"$tmpPath\"" EXIT
 
-    trap "rm -rf $tmpPath" EXIT
+    tmpFile="$tmpPath/$dstFile"
 
     # Perform the checkout.
     bzr -Ossl.cert_reqs=none export $revarg --format=dir "$tmpFile" "$url"

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -256,11 +256,11 @@ else
   # download the file and add it to the store.
   if test -z "$finalPath"; then
 
-      tmpPath=/tmp/git-checkout-tmp-$$
-      tmpFile=$tmpPath/git-export
-      mkdir $tmpPath $tmpFile
+      tmpPath="$(mktemp --tmpdir -d git-checkout-tmp-XXXXXXXX)"
+      trap "rm -rf \"$tmpPath\"" EXIT
 
-      trap "rm -rf $tmpPath" EXIT
+      tmpFile="$tmpPath/git-export"
+      mkdir "$tmpFile"
 
       # Perform the checkout.
       clone_user_rev "$tmpFile" "$url" "$rev"

pkgs/build-support/fetchhg/nix-prefetch-hg

@@ -35,11 +35,10 @@ fi
 # download the file and add it to the store.
 if test -z "$finalPath"; then
 
-    tmpPath=/tmp/hg-checkout-tmp-$$
-    tmpArchive=$tmpPath/hg-archive
-    mkdir $tmpPath
+    tmpPath="$(mktemp --tmpdir -d hg-checkout-tmp-XXXXXXXX)"
+    trap "rm -rf \"$tmpPath\"" EXIT
 
-    trap "rm -rf $tmpPath" EXIT
+    tmpArchive="$tmpPath/hg-archive"
 
     # Perform the checkout.
     if [[ $url != /* ]]; then

pkgs/build-support/fetchsvn/nix-prefetch-svn

@@ -41,11 +41,10 @@ fi
 # If we don't know the hash or a path with that hash doesn't exist,
 # download the file and add it to the store.
 if test -z "$finalPath"; then
-    tmpPath=/tmp/svn-checkout-tmp-$$
-    tmpFile=$tmpPath/$dstFile
-    mkdir $tmpPath
+    tmpPath="$(mktemp --tmpdir -d svn-checkout-tmp-XXXXXXXX)"
+    trap "rm -rf \"$tmpPath\"" EXIT
 
-    trap "rm -rf $tmpPath" EXIT
+    tmpFile="$tmpPath/$dstFile"
 
     # Perform the checkout.
     if test "$NIX_PREFETCH_SVN_LEAVE_DOT_SVN" != 1