new file mode 100644
--- /dev/null
+++ b/src/logfile.c
@@ -0,0 +1,208 @@
+/* vi:set ts=8 sts=4 sw=4 noet:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ */
+
+/*
+ * Implements logging.  Originally intended for the channel feature, which is
+ * why the "ch_" prefix is used.  Also useful for any kind of low-level and
+ * async debuging.
+ */
+
+#include "vim.h"
+
+#if defined(FEAT_EVAL) || defined(PROTO)
+
+// Log file opened with ch_logfile().
+static FILE *log_fd = NULL;
+static char_u *log_name = NULL;
+#ifdef FEAT_RELTIME
+static proftime_T log_start;
+#endif
+
+    void
+ch_logfile(char_u *fname, char_u *opt)
+{
+    FILE	*file = NULL;
+    char	*mode = "a";
+
+    if (log_fd != NULL)
+    {
+	if (*fname != NUL)
+	    ch_log(NULL, "closing this logfile, opening %s", fname);
+	else
+	    ch_log(NULL, "closing logfile %s", log_name);
+	fclose(log_fd);
+    }
+
+    // The "a" flag overrules the "w" flag.
+    if (vim_strchr(opt, 'a') == NULL && vim_strchr(opt, 'w') != NULL)
+	mode = "w";
+    ch_log_output = vim_strchr(opt, 'o') != NULL ? LOG_ALWAYS : FALSE;
+
+    if (*fname != NUL)
+    {
+	file = fopen((char *)fname, mode);
+	if (file == NULL)
+	{
+	    semsg(_(e_cant_open_file_str), fname);
+	    return;
+	}
+	vim_free(log_name);
+	log_name = vim_strsave(fname);
+    }
+    log_fd = file;
+
+    if (log_fd != NULL)
+    {
+	fprintf(log_fd, "==== start log session %s ====\n",
+						 get_ctime(time(NULL), FALSE));
+	// flush now, if fork/exec follows it could be written twice
+	fflush(log_fd);
+#ifdef FEAT_RELTIME
+	profile_start(&log_start);
+#endif
+    }
+}
+
+    int
+ch_log_active(void)
+{
+    return log_fd != NULL;
+}
+
+    static void
+ch_log_lead(const char *what, channel_T *ch, ch_part_T part)
+{
+    if (log_fd == NULL)
+	return;
+
+#ifdef FEAT_RELTIME
+    proftime_T log_now;
+
+    profile_start(&log_now);
+    profile_sub(&log_now, &log_start);
+    fprintf(log_fd, "%s ", profile_msg(&log_now));
+#endif
+#ifdef FEAT_JOB_CHANNEL
+    if (ch != NULL)
+    {
+	if (part < PART_COUNT)
+	    fprintf(log_fd, "%son %d(%s): ", what, ch->ch_id, ch_part_names[part]);
+	else
+	    fprintf(log_fd, "%son %d: ", what, ch->ch_id);
+    }
+    else
+#endif
+	fprintf(log_fd, "%s: ", what);
+}
+
+#ifndef PROTO  // prototype is in proto.h
+    void
+ch_log(channel_T *ch, const char *fmt, ...)
+{
+    if (log_fd == NULL)
+	return;
+
+    va_list ap;
+
+    ch_log_lead("", ch, PART_COUNT);
+    va_start(ap, fmt);
+    vfprintf(log_fd, fmt, ap);
+    va_end(ap);
+    fputc('\n', log_fd);
+    fflush(log_fd);
+    did_repeated_msg = 0;
+}
+
+    void
+ch_error(channel_T *ch, const char *fmt, ...)
+{
+    if (log_fd == NULL)
+	return;
+
+    va_list ap;
+
+    ch_log_lead("ERR ", ch, PART_COUNT);
+    va_start(ap, fmt);
+    vfprintf(log_fd, fmt, ap);
+    va_end(ap);
+    fputc('\n', log_fd);
+    fflush(log_fd);
+    did_repeated_msg = 0;
+}
+#endif
+
+#if defined(FEAT_JOB_CHANNEL) || defined(PROTO)
+/*
+ * Log a message "buf[len]" for channel "ch" part "part".
+ * Only to be called when ch_log_active() returns TRUE.
+ */
+    void
+ch_log_literal(
+	char	    *lead,
+	channel_T   *ch,
+	ch_part_T   part,
+	char_u	    *buf,
+	int	    len)
+{
+    ch_log_lead(lead, ch, part);
+    fprintf(log_fd, "'");
+    vim_ignored = (int)fwrite(buf, len, 1, log_fd);
+    fprintf(log_fd, "'\n");
+    fflush(log_fd);
+}
+#endif
+
+/*
+ * "ch_log()" function
+ */
+    void
+f_ch_log(typval_T *argvars, typval_T *rettv UNUSED)
+{
+    char_u	*msg;
+    channel_T	*channel = NULL;
+
+    if (in_vim9script()
+	    && (check_for_string_arg(argvars, 0) == FAIL
+		|| check_for_opt_chan_or_job_arg(argvars, 1) == FAIL))
+	return;
+
+    msg = tv_get_string(&argvars[0]);
+#if defined(FEAT_JOB_CHANNEL)
+    if (argvars[1].v_type != VAR_UNKNOWN)
+	channel = get_channel_arg(&argvars[1], FALSE, FALSE, 0);
+#endif
+
+    ch_log(channel, "%s", msg);
+}
+
+/*
+ * "ch_logfile()" function
+ */
+    void
+f_ch_logfile(typval_T *argvars, typval_T *rettv UNUSED)
+{
+    char_u *fname;
+    char_u *opt = (char_u *)"";
+    char_u buf[NUMBUFLEN];
+
+    // Don't open a file in restricted mode.
+    if (check_restricted() || check_secure())
+	return;
+
+    if (in_vim9script()
+	    && (check_for_string_arg(argvars, 0) == FAIL
+		|| check_for_opt_string_arg(argvars, 1) == FAIL))
+	return;
+
+    fname = tv_get_string(&argvars[0]);
+    if (argvars[1].v_type == VAR_STRING)
+	opt = tv_get_string_buf(&argvars[1], buf);
+    ch_logfile(fname, opt);
+}
+
+#endif // FEAT_EVAL