Add exec/2: posix_spawn

This is an initial MVP with quite a few things still missing (such as:
better error messages, documentation, tests).
Despite this, it is already feature-complete on POSIX platforms (on
Windows it currently reports an "unsupported on this platform" error).

The signature is `anything | exec(path; [args…])`.
`path` and all `args` must be strings.
`anything` will be converted to a string if it isn't a string in memory,
then piped into the process' stdin.
The output is all stdout of the process.
The exit code is not reported.

Technically, "path" can be a simple name and `$PATH` will be searched.
This is because the underlying function is `posix_spawnp`.
This can bec hanged easily.

The process does not have access to environment variables.
This can be changed as well.

Piping between programs works. Here's an example to try it out:
`tostring | exec("seq"; [.]) | exec("wc"; "-l")`
Expected output when inputting numbers is that number,
but it notably goes through seq, then line-counting.

src/builtin.c

@@ -34,6 +34,10 @@ void *alloca (size_t);
 #include <time.h>
 #ifdef WIN32
 #include <windows.h>
+#else
+#include <spawn.h>
+#include <sys/wait.h>
+#include <unistd.h>
 #endif
 #include "builtin.h"
 #include "compile.h"
@@ -1750,6 +1754,161 @@ static jv f_have_decnum(jq_state *jq, jv a) {
 #endif
 }
 
+#ifdef WIN32
+static jv f_exec(jq_state *jq, jv input, jv path, jv args) {
+	jv_free(input), jv_free(path), jv_free(args);
+  return jv_invalid_with_msg(jv_string("exec not supported on this platform"));
+}
+#else
+static jv f_exec(jq_state *jq, jv input, jv path, jv args) {
+	int ret = 0;
+
+	/* argument validation */
+	if (jv_get_kind(path) != JV_KIND_STRING) {
+		jv_free(input), jv_free(path), jv_free(args);
+		return type_error(path, "exec/2: path must be string");
+	}
+
+	// extract args into const char ** on the stack
+	if (jv_get_kind(args) != JV_KIND_ARRAY) {
+		jv_free(input), jv_free(path), jv_free(args);
+		return type_error(args, "exec/2: args must be array");
+	}
+
+	// validate args array before using it to avoid having to clean up
+	// a partially populated argv
+	jv_array_foreach(args, i, s) {
+		if (jv_get_kind(s) != JV_KIND_STRING) ret++;
+		jv_free(s);
+	}
+	if (ret) {
+		jv_free(input), jv_free(path), jv_free(args);
+		return type_error(args, "exec/2: args must only contain strings");
+	}
+
+	const size_t argc = jv_array_length(jv_copy(args)) + 1;
+	// this can't be a * const because of how we initialize it
+	char * argv[argc + 1];
+	jv_array_foreach(args, i, s) {
+		argv[i + 1] = strdup(jv_string_value(s));
+		jv_free(s);
+	}
+	argv[0] = strdup(jv_string_value(path));
+	argv[argc] = 0;
+	jv_free(path);
+
+	/* setting up pipes */
+	int fin[2] = {0, 0}, fout[2] = {0, 0};
+	posix_spawn_file_actions_t fda;
+	if ((ret = posix_spawn_file_actions_init(&fda))) {
+		jv_free(args), jv_free(input);
+		return jv_invalid_with_msg(jv_string("exec/2: could not initialize fd actions"));
+	}
+
+	// TODO: better error reporting
+	if ((ret = pipe(fin))) {
+		jv_free(args), jv_free(input);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_addclose(&fda, fin[1]))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_adddup2(&fda, fin[0], 0))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_addclose(&fda, fin[0]))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+
+	// TODO: better error reporting
+	if ((ret = pipe(fout))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_addclose(&fda, fout[0]))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		close(fout[0]), close(fout[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_adddup2(&fda, fout[1], 1))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		close(fout[0]), close(fout[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	if ((ret = posix_spawn_file_actions_addclose(&fda, fout[1]))) {
+		jv_free(args), jv_free(input);
+		close(fin[0]), close(fin[1]);
+		close(fout[0]), close(fout[1]);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+
+	/* execute */
+	pid_t pid;
+	// NOTE: the warning on argv should be fine, posix_spawnp doesn't mutate those to my knowledge
+	if (posix_spawnp(&pid, argv[0], &fda,
+				NULL, argv, NULL)) {
+		close(fin[0]), close(fin[1]);
+		close(fout[0]), close(fout[1]);
+		jv_free(input);
+		return jv_invalid_with_msg(jv_string("exec/2: PLACEHOLDER"));
+	}
+	for (size_t i = 0; i < argc; i++) {
+		free(argv[i]);
+	}
+	close(fin[0]), close(fout[1]);
+	jv_free(args);
+	if ((ret = posix_spawn_file_actions_destroy(&fda))) {
+		// TODO: what should we do here? this is technically harmless
+	}
+
+	/* send and receive data */
+	// TODO: error checking on the writes
+	switch (jv_get_kind(input)) {
+		case JV_KIND_INVALID:
+		case JV_KIND_NULL:
+			close(fin[1]);
+			jv_free(input);
+			break; // do not pipe invalid / null
+		case JV_KIND_STRING:
+			write(fin[1], jv_string_value(input), jv_string_length_bytes(jv_copy(input)));
+			close(fin[1]);
+			jv_free(input);
+			break;
+		default: {
+			jv s = jv_dump_string(input, 0);
+			write(fin[1], jv_string_value(s), jv_string_length_bytes(jv_copy(s)));
+			close(fin[1]);
+			jv_free(s);
+			break;
+		}
+	}
+
+	jv output = jv_string_empty(0);
+	char *buf = malloc(1024);
+	ssize_t bytes;
+	while ((bytes = read(fout[0], buf, 1024)) > 0) {
+		output = jv_string_append_buf(output, buf, bytes);
+	}
+	close(fout[0]);
+	free(buf);
+
+	// TODO: parse output into json? probably not.
+
+	// TODO: check waitpid output
+	waitpid(pid, &ret, 0);
+	return output;
+}
+#endif
+
 #define LIBM_DD(name) \
   {f_ ## name, #name, 1},
 #define LIBM_DD_NO(name) LIBM_DD(name)
@@ -1829,6 +1988,7 @@ BINOPS
   {f_current_line, "input_line_number", 1},
   {f_have_decnum, "have_decnum", 1},
   {f_have_decnum, "have_literal_numbers", 1},
+  {f_exec, "exec", 3},
 };
 #undef LIBM_DDDD_NO
 #undef LIBM_DDD_NO