Incorrect cursor placement in minibuffer when prefixed

[Raphus-cucullatus]

I sometimes want to pass additional argument to ag. According to the DOCSTRING,

If called with a prefix, prompts for flags to pass to ag.

It works well when my search string contains no shell-quote characters. For example, if I use C-u M-x ag, and search "struct", command args will be prompted in the minibuffer with the point/cursor placed at the end of other args and before the "--" and wait for my input (Here I use <cursor> to indicate the cursor position):

ag command: ag --literal --group --line-number --column --color --color-match 30\;43 --color-path 1\;32 --smart-case --stats <cursor> -- struct .

However, if my search string contains shell-quote characters, the cursor is placed weirdly:

1. if I use C-u M-x ag, and search "struct kvm":

ag command: ag --literal --group --line-number --column --color --color-match 30\;43 --color-path 1\;32 --smart-case --stats  <cursor>-- struct\ kvm .

2. if there are more shell-quote characters, for example, search "struct kvm {":

ag command: ag --literal --group --line-number --column --color --color-match 30\;43 --color-path 1\;32 --smart-case --stats  --<cursor> struct\ kvm\ \{ .

The cursor is placed after the "--"!

If I want to add additional argument, I have to move the cursor back before "--". It seems that the cursor position in the minibuffer is incorrectly calculated.

My guess

In ag/search, the point position is calculated as follows:

ag.el/ag.el

Line 262 in bd81d68

(let ((adjusted-point (- (length command-string) (length string) 5)))

However, here command-string is quoted, while string is unquoted. So direct subtraction does not take the backslash into account. In my case, command-string is ag --literal --group ... struct\ kvm\ \{ ., while string is struct kvm {.

Subtracting by 5 is also incorrect after the introduction of the commit bd81d68 since the length after "--" can be arbitrary due to :files.

Other Information

On my machine, the output of ag --version is

ag version 2.2.0

Features:
  +jit +lzma +zlib

My Emacs version is 26.1 (Download from emacsformacosx).

I'm using: macOS Mojave 10.14.6.