Proceed when target is absent; account for merges

This modifies the script so that:

- When the target does not exist on disk at all, we proceed, even
  if the deletion is an unstaged change.

- When there is an uncommitted merge, as happens when a conflict is
  being resolved (and occasionally otherwise), recognize this case
  (rather than misinterpreting the short status output), proceeding
  only if there are no changes at all at the target. Even staged
  changes cause the copy to be cancelled in that case.

- Some cumbersomely expressed code is written more clearly, a
  couple of specific error messages are added for what should be
  rare failures of git commands, checking for grep failure is
  removed (grep should not actually fail if used correctly), and
  the big comment in first_line_ends_crlf is made more accurate.

Author: EliahKagan

etc/copy-packetline.sh

@@ -16,26 +16,58 @@ function fail () {
   exit 1
 }
 
-function status () {
-  git status --short --ignored=traditional -- "$@"
+function chdir_toplevel() {
+  local root
+
+  # NOTE: We get the wrong directory name, if the name ends in newline.
+  root="$(git rev-parse --show-toplevel)" ||
+    fail 'git-rev-parse failed to find top-level dir'
+
+  cd -- "$root"
+}
+
+function merging () {
+  local git_dir
+
+  # NOTE: We get the wrong directory name, if the name ends in newline.
+  git_dir="$(git rev-parse --git-dir)" ||
+    fail 'git-rev-parse failed to find git dir'
+
+  test -e "$git_dir/MERGE_HEAD"
+}
+
+function target_status () {
+  git status --short --ignored=traditional -- gix-packetline-blocking/src ||
+    fail 'git-status failed'
 }
 
-function avoids_pattern () (
-  set +e  # Temporary, since the function body is in ( ).
-  grep -q "$@"
-  test "$?" -eq 1  # Distinguish no-match from error.
-)
+function check_target () {
+  if ! test -e "gix-packetline-blocking/src"; then
+    # The target does not exist on disk, so nothing will be lost. Proceed.
+    return
+  fi
+
+  if merging; then
+    # In a merge, it would be confusing to replace anything at the target.
+    if target_status | grep -q '^'; then
+      fail 'target exists, and a merge is in progress'
+    fi
+  else
+    # We can lose data if anything of value at the target is not in the index.
+    if target_status | grep -q '^.[^ ]'; then
+      fail 'target exists, with unstaged changes or ignored files'
+    fi
+  fi
+}
 
 function indent () {
   sed 's/^/    /'
 }
 
 function generate_all () {
-  local root failures
+  local failures
 
-  # Find the working tree. (NOTE: Wrong if the directory name ends in newline.)
-  root="$(git rev-parse --show-toplevel)"
-  cd -- "$root"
+  chdir_toplevel
 
   if ! test -d gix-packetline/src; then
     fail 'no source directory: gix-packetline/src'
@@ -44,12 +76,9 @@ function generate_all () {
     fail 'no target parent directory: gix-packetline-blocking'
   fi
 
-  # FIXME: This misinterprets the status when in an unresolved merge conflict!
-  if ! status gix-packetline-blocking/src | avoids_pattern '^.[^ ]'; then
-    fail 'target has unstaged changes or contains ignored files'
-  fi
+  check_target
 
-  rm -rf gix-packetline-blocking/src  # No trailing /, as it may be a symlink.
+  rm -rf gix-packetline-blocking/src  # No trailing /. It may be a symlink.
   if test -e gix-packetline-blocking/src; then
     fail 'unable to remove target'
   fi
@@ -67,16 +96,17 @@ function generate_all () {
 }
 
 function first_line_ends_crlf () {
-  # This is tricky to check portably. On Windows in Cygwin-like environments
-  # including MSYS2 and Git Bash, most text processing tools, including awk,
-  # sed, and grep, automatically substitute \n for \r\n. Some can be told not
-  # to, but in non-portable ways that may affect other implementations. Bash
-  # does so on command substitution and other input, and optionally more often.
-  # Easy ways to check are often non-portable to other OSes. Fortunately, tools
-  # that treat input as binary data are exempt (including cat, but "-v" is not
-  # portable, and it's unreliable in general as lines can end in "^M"). This
-  # may be doable without od, by using tr more heavily, but it could be hard to
-  # avoid false positives with unexpected characters, or with \r not before \n.
+  # This is tricky to check portably. In Cygwin-like environments including
+  # MSYS2 and Git Bash, most text processing tools, including awk, sed, and
+  # grep, automatically ignore \r before \n. Some ignore \r everywhere. Some
+  # can be told to keep \r, but in non-portable ways that may affect other
+  # implementations. Bash ignores \r in some places even without "-o icncr",
+  # and ignores \r even more with it, including in all text from command
+  # substitution. Simple checks may be non-portable to other OSes. Fortunately,
+  # tools that treat input as binary data are exempt (even cat, but "-v" is
+  # non-portable, and unreliable in general because lines can end in "^M").
+  # This may be doable without od, by using tr more heavily, but it could be
+  # hard to avoid false positives with unexpected characters or \r without \n.
 
   head -n 1 -- "$1" |  # Get the longest prefix with no non-trailing \n byte.
     od -An -ta |       # Represent all bytes symbolically, without addresses.