patchShebangs: rewrite ".../bin/env python" to /nix/store/.../python

Currently "/usr/bin/env python" is rewritten to "/nix/store/.../env
python". That doesn't really improve anything because the interpreter
still have to be located in $PATH at runtime. The result is that many
nix package expressions do .../bin/env fixup themselves.

Instead of everyone having to do this patching locally, add the
functionality to the standard environment patchShebangs function so that
everyone can benefit.

pkgs/stdenv/generic/setup.sh

@@ -655,20 +655,57 @@ patchShebangs() {
     # Rewrite all script interpreter file names (`#! /path') under the
     # specified  directory tree to paths found in $PATH.  E.g.,
     # /bin/sh will be rewritten to /nix/store/<hash>-some-bash/bin/sh.
+    # /usr/bin/env gets special treatment so that ".../bin/env python" is
+    # rewritten to /nix/store/<hash>/bin/python.
     # Interpreters that are already in the store are left untouched.
     header "patching script interpreter paths"
     local dir="$1"
     local f
+    local oldPath
+    local newPath
+    local arg0
+    local args
+    local oldInterpreterLine
+    local newInterpreterLine
+
     for f in $(find "$dir" -type f -perm +0100); do
-        local oldPath=$(sed -ne '1 s,^#![ ]*\([^ ]*\).*$,\1,p' "$f")
+        if [ $(head -1 "$f" | head -c +2) != '#!' ]; then
+            # missing shebang => not a script
+            continue
+        fi
+
+        oldInterpreterLine=$(head -1 "$f" | tail -c +3)
+        read oldPath arg0 args <<< "$oldInterpreterLine"
+
+        if $(echo "$oldPath" | grep -q "/bin/env$"); then
+            # Check for unsupported 'env' functionality:
+            # - options: something starting with a '-'
+            # - environment variables: foo=bar
+            if $(echo "$arg0" | grep -q -- "^-.*\|.*=.*"); then
+                echo "unsupported interpreter directive \"$oldInterpreterLine\" (set dontPatchShebangs=1 and handle shebang patching yourself)"
+                exit 1
+            fi
+            newPath="$(command -v "$arg0")"
+        else
+            if [ "$oldPath" = "" ]; then
+                # If no interpreter is specified linux will use /bin/sh. Set
+                # oldpath="/bin/sh" so that we get /nix/store/.../sh.
+                oldPath="/bin/sh"
+            fi
+            newPath="$(command -v "$(basename "$oldPath")")"
+            args="$arg0 $args"
+        fi
+
+        newInterpreterLine="$newPath $args"
+
         if [ -n "$oldPath" -a "${oldPath:0:${#NIX_STORE}}" != "$NIX_STORE" ]; then
-            local newPath=$(type -P $(basename $oldPath) || true)
             if [ -n "$newPath" -a "$newPath" != "$oldPath" ]; then
-                echo "$f: interpreter changed from $oldPath to $newPath"
+                echo "$f: interpreter directive changed from \"$oldInterpreterLine\" to \"$newInterpreterLine\""
-                sed -i -e "1 s,$oldPath,$newPath," "$f"
+                sed -i -e "1 s|.*|#\!$newInterpreterLine|" "$f"
             fi
         fi
     done
+
     stopNest
 }