fetchpatch: fail on empty patch

Since this is probably never the desired case and has led to actual
issues, see the comments at:

af1313e915

This might also happen when pulling a patch from GitHub or a similar web
interface without explicitly selecting the "raw" format.

pkgs/build-support/fetchpatch/default.nix

@@ -10,6 +10,10 @@
 fetchurl ({
   postFetch = ''
     tmpfile="$TMPDIR/${args.sha256}"
+    if [ ! -s "$out" ]; then
+      echo "error: Fetched patch file '$out' is empty!" 1>&2
+      exit 1
+    fi
     "${patchutils}/bin/lsdiff" "$out" \
       | sort -u | sed -e 's/[*?]/\\&/g' \
       | xargs -I{} \
@@ -21,12 +25,26 @@ fetchurl ({
            --addnewprefix=b/${extraPrefix} \
         ''} \
         --clean "$out" > "$tmpfile"
+    if [ ! -s "$tmpfile" ]; then
+      echo "error: Normalized patch '$tmpfile' is empty (while the fetched file was not)!" 1>&2
+      echo "Did you maybe fetch a HTML representation of a patch instead of a raw patch?" 1>&2
+      echo "Fetched file was:" 1>&2
+      cat "$out" 1>&2
+      exit 1
+    fi
     ${patchutils}/bin/filterdiff \
       -p1 \
       ${builtins.toString (builtins.map (x: "-x ${lib.escapeShellArg x}") excludes)} \
       ${builtins.toString (builtins.map (x: "-i ${lib.escapeShellArg x}") includes)} \
       "$tmpfile" > "$out"
     ${args.postFetch or ""}
+    if [ ! -s "$out" ]; then
+      echo "error: Filtered patch '$out$' is empty (while the original patch file was not)!" 1>&2
+      echo "Check your includes and excludes." 1>&2
+      echo "Normalizd patch file was:" 1>&2
+      cat "$tmpfile" 1>&2
+      exit 1
+    fi
   '';
   meta.broken = excludes != [] && includes != [];
 } // builtins.removeAttrs args ["stripLen" "extraPrefix" "excludes" "includes" "postFetch"])