Flake8 diagnostics support

This follows the clang diagnostics example and does the same thing. It
just uses the flake8 program and the regex's are a bit different.

rc/extra/flake8.kak

@@ -0,0 +1,83 @@
+decl str flake8_options
+decl -hidden str flake8_tmp_dir
+decl -hidden line-flags flake8_flags
+decl -hidden str flake8_errors
+
+def flake8-lint -params 0..1 -docstring "Lint the contents of the current buffer with flake8" %{
+    %sh{
+        dir=$(mktemp -d -t kak-flake8.XXXXXXXX)
+        mkfifo ${dir}/fifo
+        echo "set buffer flake8_tmp_dir ${dir}"
+        echo "write ${dir}/buf"
+    }
+    # end the previous %sh{} so that its output gets interpreted by kakoune
+    # before launching the following as a background task.
+    %sh{
+        dir=${kak_opt_flake8_tmp_dir}
+        echo "eval -draft %{
+                  edit! -fifo ${dir}/fifo *flake8-output*
+                  set buffer filetype make
+                  set buffer _make_current_error_line 0
+                  hook -group fifo buffer BufCloseFifo .* %{
+                      nop %sh{ rm -r ${dir} }
+                      rmhooks buffer fifo
+                  }
+              }"
+        # this runs in a detached shell, asynchronously, so that kakoune does
+        # not hang while flake8 is running.
+		(
+			flake8 --ignore=${kak_opt_flake8_options} - < ${dir}/buf > ${dir}/stderr
+
+            flags=$(cat ${dir}/stderr | sed -rne "
+						/^stdin:[0-9]+:[0-9]+:? (W|F|C|N).*/ { s/^stdin:([0-9]+):.*/\1|{yellow}█/; p }
+						/^stdin:[0-9]+:[0-9]+:? E.*/ { s/^stdin:([0-9]+):.*/\1|{red}█/; p }
+                    " | paste -s -d ':')
+
+            errors=$(cat ${dir}/stderr | sed -rne "
+						/^stdin:[0-9]+:[0-9]+:?.*/ { s/^stdin:([0-9]+):([0-9]+:)? (.*) /\1,\3/; s/'/\\\\'/g; p }
+                     " | sort -n)
+
+            sed -e "s|<stdin>|${kak_bufname}|g" < ${dir}/stderr > ${dir}/fifo
+
+            echo "set 'buffer=${kak_buffile}' flake8_flags %{${kak_timestamp}:${flags}}
+                  set 'buffer=${kak_buffile}' flake8_errors '${errors}'" | kak -p ${kak_session}
+        ) > /dev/null 2>&1 < /dev/null &
+    }
+}
+
+def -hidden flake8-show-error-info %{ %sh{
+    desc=$(printf %s "${kak_opt_flake8_errors}" | sed -ne "/^${kak_cursor_line},.*/ { s/^[[:digit:]]\+,//g; s/'/\\\\'/g; p }")
+    if [ -n "$desc" ]; then
+        echo "info -anchor ${kak_cursor_line}.${kak_cursor_column} '${desc}'"
+    fi
+} }
+
+def flake8-enable-diagnostics -docstring "Activate automatic diagnostics of the code by flake8" %{
+    addhl flag_lines default flake8_flags
+    hook window -group flake8-diagnostics NormalIdle .* %{ flake8-show-error-info }
+}
+
+def flake8-disable-diagnostics -docstring "Disable automatic diagnostics of the code" %{
+    rmhl hlflags_flake8_flags
+    rmhooks window flake8-diagnostics
+}
+
+def flake8-diagnostics-next -docstring "Jump to the next line that contains an error" %{ %sh{
+    printf "%s\n" "${kak_opt_flake8_errors}" | (
+        line=-1
+        first_line=-1
+        while read line_content; do
+            candidate=${line_content%%,*}
+            if [ -n "$candidate" ]; then
+                first_line=$(( first_line == -1 ? candidate : first_line ))
+                line=$((candidate > kak_cursor_line && (candidate < line || line == -1) ? candidate : line ))
+            fi
+        done
+        line=$((line == -1 ? first_line : line))
+        if [ ${line} -ne -1 ]; then
+            echo "exec ${line} g"
+        else
+            echo 'echo -color Error no next flake8 diagnostic'
+        fi
+    )
+} }