[PATCH v2 8/9] built-in add -p: handle Escape sequences more efficiently

From: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>
Date: Dec 25, 2019, 11:56 UTC
Message-ID: <703719ffce4e69fa1d22fd2b740ddfaa0ef7283d.1577275020.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.175.v2.git.1577275020.gitgitgadget@gmail.com>

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When `interactive.singlekey = true`, we react immediately to keystrokes, even to Escape sequences (e.g. when pressing a cursor key).

The problem with Escape sequences is that we do not really know when they are done, and as a heuristic we poll standard input for half a second to make sure that we got all of it.

While waiting half a second is not asking for a whole lot, it can become quite annoying over time, therefore with this patch, we read the terminal capabilities (if available) and extract known Escape sequences from there, then stop polling immediately when we detected that the user pressed a key that generated such a known sequence.

This recapitulates the remaining part of b5cc003253c8 (add -i: ignore terminal escape sequences, 2011-05-17).

Note: We do *not* query the terminal capabilities directly. That would
either require a lot of platform-specific code, or it would require
linking to a library such as ncurses.

Linking to a library in the built-ins is something we try very hard to avoid (we even kicked the libcurl dependency to a non-built-in remote helper, just to shave off a tiny fraction of a second from Git's startup time). And the platform-specific code would be a maintenance nightmare.

Even worse: in Git for Windows' case, we would need to query MSYS2 pseudo terminals, which `git.exe` simply cannot do (because it is intentionally *not* an MSYS2 program).

To address this, we simply spawn `infocmp -L -1` and parse its output (which works even in Git for Windows, because that helper is included in the end-user facing installations).

This is done only once, as in the Perl version, but it is done only when the first Escape sequence is encountered, not upon startup of `git add -i`; This saves on startup time, yet makes reacting to the first Escape sequence slightly more sluggish. But it allows us to keep the terminal-related code encapsulated in the `compat/terminal.c` file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/terminal.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/compat/terminal.c b/compat/terminal.c
index b7f58d1781..35bca03d14 100644
--- a/compat/terminal.c
+++ b/compat/terminal.c
@@ -4,6 +4,7 @@
 #include "strbuf.h"
 #include "run-command.h"
 #include "string-list.h"
+#include "hashmap.h"
 
 #if defined(HAVE_DEV_TTY) || defined(GIT_WINDOWS_NATIVE)
 
@@ -238,6 +239,71 @@ char *git_terminal_prompt(const char *prompt, int echo)
 	return buf.buf;
 }
 
+/*
+ * The `is_known_escape_sequence()` function returns 1 if the passed string
+ * corresponds to an Escape sequence that the terminal capabilities contains.
+ *
+ * To avoid depending on ncurses or other platform-specific libraries, we rely
+ * on the presence of the `infocmp` executable to do the job for us (failing
+ * silently if the program is not available or refused to run).
+ */
+struct escape_sequence_entry {
+	struct hashmap_entry entry;
+	char sequence[FLEX_ARRAY];
+};
+
+static int sequence_entry_cmp(const void *hashmap_cmp_fn_data,
+			      const struct escape_sequence_entry *e1,
+			      const struct escape_sequence_entry *e2,
+			      const void *keydata)
+{
+	return strcmp(e1->sequence, keydata ? keydata : e2->sequence);
+}
+
+static int is_known_escape_sequence(const char *sequence)
+{
+	static struct hashmap sequences;
+	static int initialized;
+
+	if (!initialized) {
+		struct child_process cp = CHILD_PROCESS_INIT;
+		struct strbuf buf = STRBUF_INIT;
+		char *p, *eol;
+
+		hashmap_init(&sequences, (hashmap_cmp_fn)sequence_entry_cmp,
+			     NULL, 0);
+
+		argv_array_pushl(&cp.args, "infocmp", "-L", "-1", NULL);
+		if (pipe_command(&cp, NULL, 0, &buf, 0, NULL, 0))
+			strbuf_setlen(&buf, 0);
+
+		for (eol = p = buf.buf; *p; p = eol + 1) {
+			p = strchr(p, '=');
+			if (!p)
+				break;
+			p++;
+			eol = strchrnul(p, '\n');
+
+			if (starts_with(p, "\\E")) {
+				char *comma = memchr(p, ',', eol - p);
+				struct escape_sequence_entry *e;
+
+				p[0] = '^';
+				p[1] = '[';
+				FLEX_ALLOC_MEM(e, sequence, p, comma - p);
+				hashmap_entry_init(&e->entry,
+						   strhash(e->sequence));
+				hashmap_add(&sequences, &e->entry);
+			}
+			if (!*eol)
+				break;
+		}
+		initialized = 1;
+	}
+
+	return !!hashmap_get_from_hash(&sequences, strhash(sequence), sequence);
+}
+
 int read_key_without_echo(struct strbuf *buf)
 {
 	static int warning_displayed;
@@ -271,7 +337,12 @@ int read_key_without_echo(struct strbuf *buf)
 		 * Start by replacing the Escape byte with ^[ */
 		strbuf_splice(buf, buf->len - 1, 1, "^[", 2);
 
-		for (;;) {
+		/*
+		 * Query the terminal capabilities once about all the Escape
+		 * sequences it knows about, so that we can avoid waiting for
+		 * half a second when we know that the sequence is complete.
+		 */
+		while (!is_known_escape_sequence(buf->buf)) {
 			struct pollfd pfd = { .fd = 0, .events = POLLIN };
 
 			if (poll(&pfd, 1, 500) < 1)

-- 
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