diff --git a/Documentation/git-archive.txt b/Documentation/git-archive.txt
index 94519aae23..56989a2f34 100644
--- a/Documentation/git-archive.txt
+++ b/Documentation/git-archive.txt
@@ -51,7 +51,7 @@ OPTIONS
 --prefix=<prefix>/::
 	Prepend <prefix>/ to paths in the archive.  Can be repeated; its
 	rightmost value is used for all tracked files.  See below which
-	value gets used by `--add-file`.
+	value gets used by `--add-file` and `--add-virtual-file`.
 
 -o <file>::
 --output=<file>::
@@ -63,6 +63,23 @@ OPTIONS
 	concatenating the value of the last `--prefix` option (if any)
 	before this `--add-file` and the basename of <file>.
 
+--add-virtual-file=<path>:<content>::
+	Add the specified contents to the archive.  Can be repeated to add
+	multiple files.  The path of the file in the archive is built
+	by concatenating the value of the last `--prefix` option (if any)
+	before this `--add-virtual-file` and `<path>`.
++
+The `<path>` argument can start and end with a literal double-quote
+character; the contained file name is interpreted as a C-style string,
+i.e. the backslash is interpreted as escape character. The path must
+be quoted if it contains a colon, to avoid the colon from being
+misinterpreted as the separator between the path and the contents, or
+if the path begins or ends with a double-quote character.
++
+The file mode is limited to a regular file, and the option may be
+subject to platform-dependent command-line limits. For non-trivial
+cases, write an untracked file and use `--add-file` instead.
+
 --worktree-attributes::
 	Look for attributes in .gitattributes files in the working tree
 	as well (see <<ATTRIBUTES>>).
diff --git a/archive.c b/archive.c
index e2121ebefb..d5109abb89 100644
--- a/archive.c
+++ b/archive.c
@@ -9,6 +9,7 @@
 #include "parse-options.h"
 #include "unpack-trees.h"
 #include "dir.h"
+#include "quote.h"
 
 static char const * const archive_usage[] = {
 	N_("git archive [<options>] <tree-ish> [<path>...]"),
@@ -263,6 +264,7 @@ static int queue_or_write_archive_entry(const struct object_id *oid,
 struct extra_file_info {
 	char *base;
 	struct stat stat;
+	void *content;
 };
 
 int write_archive_entries(struct archiver_args *args,
@@ -331,19 +333,27 @@ int write_archive_entries(struct archiver_args *args,
 
 		put_be64(fake_oid.hash, i + 1);
 
-		strbuf_reset(&path_in_archive);
-		if (info->base)
-			strbuf_addstr(&path_in_archive, info->base);
-		strbuf_addstr(&path_in_archive, basename(path));
+		if (!info->content) {
+			strbuf_reset(&path_in_archive);
+			if (info->base)
+				strbuf_addstr(&path_in_archive, info->base);
+			strbuf_addstr(&path_in_archive, basename(path));
 
-		strbuf_reset(&content);
-		if (strbuf_read_file(&content, path, info->stat.st_size) < 0)
-			err = error_errno(_("cannot read '%s'"), path);
-		else
-			err = write_entry(args, &fake_oid, path_in_archive.buf,
-					  path_in_archive.len,
+			strbuf_reset(&content);
+			if (strbuf_read_file(&content, path, info->stat.st_size) < 0)
+				err = error_errno(_("cannot read '%s'"), path);
+			else
+				err = write_entry(args, &fake_oid, path_in_archive.buf,
+						  path_in_archive.len,
+						  canon_mode(info->stat.st_mode),
+						  content.buf, content.len);
+		} else {
+			err = write_entry(args, &fake_oid,
+					  path, strlen(path),
 					  canon_mode(info->stat.st_mode),
-					  content.buf, content.len);
+					  info->content, info->stat.st_size);
+		}
+
 		if (err)
 			break;
 	}
@@ -493,6 +503,7 @@ static void extra_file_info_clear(void *util, const char *str)
 {
 	struct extra_file_info *info = util;
 	free(info->base);
+	free(info->content);
 	free(info);
 }
 
@@ -514,14 +525,49 @@ static int add_file_cb(const struct option *opt, const char *arg, int unset)
 	if (!arg)
 		return -1;
 
-	path = prefix_filename(args->prefix, arg);
-	item = string_list_append_nodup(&args->extra_files, path);
-	item->util = info = xmalloc(sizeof(*info));
+	info = xmalloc(sizeof(*info));
 	info->base = xstrdup_or_null(base);
-	if (stat(path, &info->stat))
-		die(_("File not found: %s"), path);
-	if (!S_ISREG(info->stat.st_mode))
-		die(_("Not a regular file: %s"), path);
+
+	if (!strcmp(opt->long_name, "add-file")) {
+		path = prefix_filename(args->prefix, arg);
+		if (stat(path, &info->stat))
+			die(_("File not found: %s"), path);
+		if (!S_ISREG(info->stat.st_mode))
+			die(_("Not a regular file: %s"), path);
+		info->content = NULL; /* read the file later */
+	} else if (!strcmp(opt->long_name, "add-virtual-file")) {
+		struct strbuf buf = STRBUF_INIT;
+		const char *p = arg;
+
+		if (*p != '"')
+			p = strchr(p, ':');
+		else if (unquote_c_style(&buf, p, &p) < 0)
+			die(_("unclosed quote: '%s'"), arg);
+
+		if (!p || *p != ':')
+			die(_("missing colon: '%s'"), arg);
+
+		if (p == arg)
+			die(_("empty file name: '%s'"), arg);
+
+		path = buf.len ?
+			strbuf_detach(&buf, NULL) : xstrndup(arg, p - arg);
+
+		if (args->prefix) {
+			char *save = path;
+			path = prefix_filename(args->prefix, path);
+			free(save);
+		}
+		memset(&info->stat, 0, sizeof(info->stat));
+		info->stat.st_mode = S_IFREG | 0644;
+		info->content = xstrdup(p + 1);
+		info->stat.st_size = strlen(info->content);
+	} else {
+		BUG("add_file_cb() called for %s", opt->long_name);
+	}
+	item = string_list_append_nodup(&args->extra_files, path);
+	item->util = info;
+
 	return 0;
 }
 
@@ -554,6 +600,9 @@ static int parse_archive_args(int argc, const char **argv,
 		{ OPTION_CALLBACK, 0, "add-file", args, N_("file"),
 		  N_("add untracked file to archive"), 0, add_file_cb,
 		  (intptr_t)&base },
+		{ OPTION_CALLBACK, 0, "add-virtual-file", args,
+		  N_("path:content"), N_("add untracked file to archive"), 0,
+		  add_file_cb, (intptr_t)&base },
 		OPT_STRING('o', "output", &output, N_("file"),
 			N_("write the archive to this file")),
 		OPT_BOOL(0, "worktree-attributes", &worktree_attributes,
diff --git a/contrib/scalar/scalar.c b/contrib/scalar/scalar.c
index 58ca0e56f1..28176914e5 100644
--- a/contrib/scalar/scalar.c
+++ b/contrib/scalar/scalar.c
@@ -11,6 +11,8 @@
 #include "dir.h"
 #include "packfile.h"
 #include "help.h"
+#include "archive.h"
+#include "object-store.h"
 
 /*
  * Remove the deepest subdirectory in the provided path string. Path must not
@@ -43,9 +45,11 @@ static void setup_enlistment_directory(int argc, const char **argv,
 		usage_with_options(usagestr, options);
 
 	/* find the worktree, determine its corresponding root */
-	if (argc == 1)
+	if (argc == 1) {
 		strbuf_add_absolute_path(&path, argv[0]);
-	else if (strbuf_getcwd(&path) < 0)
+		if (!is_directory(path.buf))
+			die(_("'%s' does not exist"), path.buf);
+	} else if (strbuf_getcwd(&path) < 0)
 		die(_("need a working directory"));
 
 	strbuf_trim_trailing_dir_sep(&path);
@@ -258,6 +262,99 @@ static int unregister_dir(void)
 	return res;
 }
 
+static int add_directory_to_archiver(struct strvec *archiver_args,
+					  const char *path, int recurse)
+{
+	int at_root = !*path;
+	DIR *dir = opendir(at_root ? "." : path);
+	struct dirent *e;
+	struct strbuf buf = STRBUF_INIT;
+	size_t len;
+	int res = 0;
+
+	if (!dir)
+		return error_errno(_("could not open directory '%s'"), path);
+
+	if (!at_root)
+		strbuf_addf(&buf, "%s/", path);
+	len = buf.len;
+	strvec_pushf(archiver_args, "--prefix=%s", buf.buf);
+
+	while (!res && (e = readdir(dir))) {
+		if (!strcmp(".", e->d_name) || !strcmp("..", e->d_name))
+			continue;
+
+		strbuf_setlen(&buf, len);
+		strbuf_addstr(&buf, e->d_name);
+
+		if (e->d_type == DT_REG)
+			strvec_pushf(archiver_args, "--add-file=%s", buf.buf);
+		else if (e->d_type != DT_DIR)
+			warning(_("skipping '%s', which is neither file nor "
+				  "directory"), buf.buf);
+		else if (recurse &&
+			 add_directory_to_archiver(archiver_args,
+						   buf.buf, recurse) < 0)
+			res = -1;
+	}
+
+	closedir(dir);
+	strbuf_release(&buf);
+	return res;
+}
+
+#ifndef WIN32
+#include <sys/statvfs.h>
+#endif
+
+static int get_disk_info(struct strbuf *out)
+{
+#ifdef WIN32
+	struct strbuf buf = STRBUF_INIT;
+	char volume_name[MAX_PATH], fs_name[MAX_PATH];
+	DWORD serial_number, component_length, flags;
+	ULARGE_INTEGER avail2caller, total, avail;
+
+	strbuf_realpath(&buf, ".", 1);
+	if (!GetDiskFreeSpaceExA(buf.buf, &avail2caller, &total, &avail)) {
+		error(_("could not determine free disk size for '%s'"),
+		      buf.buf);
+		strbuf_release(&buf);
+		return -1;
+	}
+
+	strbuf_setlen(&buf, offset_1st_component(buf.buf));
+	if (!GetVolumeInformationA(buf.buf, volume_name, sizeof(volume_name),
+				   &serial_number, &component_length, &flags,
+				   fs_name, sizeof(fs_name))) {
+		error(_("could not get info for '%s'"), buf.buf);
+		strbuf_release(&buf);
+		return -1;
+	}
+	strbuf_addf(out, "Available space on '%s': ", buf.buf);
+	strbuf_humanise_bytes(out, avail2caller.QuadPart);
+	strbuf_addch(out, '\n');
+	strbuf_release(&buf);
+#else
+	struct strbuf buf = STRBUF_INIT;
+	struct statvfs stat;
+
+	strbuf_realpath(&buf, ".", 1);
+	if (statvfs(buf.buf, &stat) < 0) {
+		error_errno(_("could not determine free disk size for '%s'"),
+			    buf.buf);
+		strbuf_release(&buf);
+		return -1;
+	}
+
+	strbuf_addf(out, "Available space on '%s': ", buf.buf);
+	strbuf_humanise_bytes(out, st_mult(stat.f_bsize, stat.f_bavail));
+	strbuf_addf(out, " (mount flags 0x%lx)\n", stat.f_flag);
+	strbuf_release(&buf);
+#endif
+	return 0;
+}
+
 /* printf-style interface, expects `<key>=<value>` argument */
 static int set_config(const char *fmt, ...)
 {
@@ -498,6 +595,196 @@ cleanup:
 	return res;
 }
 
+static void dir_file_stats_objects(const char *full_path, size_t full_path_len,
+				   const char *file_name, void *data)
+{
+	struct strbuf *buf = data;
+	struct stat st;
+
+	if (!stat(full_path, &st))
+		strbuf_addf(buf, "%-70s %16" PRIuMAX "\n", file_name,
+			    (uintmax_t)st.st_size);
+}
+
+static int dir_file_stats(struct object_directory *object_dir, void *data)
+{
+	struct strbuf *buf = data;
+
+	strbuf_addf(buf, "Contents of %s:\n", object_dir->path);
+
+	for_each_file_in_pack_dir(object_dir->path, dir_file_stats_objects,
+				  data);
+
+	return 0;
+}
+
+static int count_files(char *path)
+{
+	DIR *dir = opendir(path);
+	struct dirent *e;
+	int count = 0;
+
+	if (!dir)
+		return 0;
+
+	while ((e = readdir(dir)) != NULL)
+		if (!is_dot_or_dotdot(e->d_name) && e->d_type == DT_REG)
+			count++;
+
+	closedir(dir);
+	return count;
+}
+
+static void loose_objs_stats(struct strbuf *buf, const char *path)
+{
+	DIR *dir = opendir(path);
+	struct dirent *e;
+	int count;
+	int total = 0;
+	unsigned char c;
+	struct strbuf count_path = STRBUF_INIT;
+	size_t base_path_len;
+
+	if (!dir)
+		return;
+
+	strbuf_addstr(buf, "Object directory stats for ");
+	strbuf_add_absolute_path(buf, path);
+	strbuf_addstr(buf, ":\n");
+
+	strbuf_add_absolute_path(&count_path, path);
+	strbuf_addch(&count_path, '/');
+	base_path_len = count_path.len;
+
+	while ((e = readdir(dir)) != NULL)
+		if (!is_dot_or_dotdot(e->d_name) &&
+		    e->d_type == DT_DIR && strlen(e->d_name) == 2 &&
+		    !hex_to_bytes(&c, e->d_name, 1)) {
+			strbuf_setlen(&count_path, base_path_len);
+			strbuf_addstr(&count_path, e->d_name);
+			total += (count = count_files(count_path.buf));
+			strbuf_addf(buf, "%s : %7d files\n", e->d_name, count);
+		}
+
+	strbuf_addf(buf, "Total: %d loose objects", total);
+
+	strbuf_release(&count_path);
+	closedir(dir);
+}
+
+static int cmd_diagnose(int argc, const char **argv)
+{
+	struct option options[] = {
+		OPT_END(),
+	};
+	const char * const usage[] = {
+		N_("scalar diagnose [<enlistment>]"),
+		NULL
+	};
+	struct strbuf zip_path = STRBUF_INIT;
+	struct strvec archiver_args = STRVEC_INIT;
+	char **argv_copy = NULL;
+	int stdout_fd = -1, archiver_fd = -1;
+	time_t now = time(NULL);
+	struct tm tm;
+	struct strbuf path = STRBUF_INIT, buf = STRBUF_INIT;
+	int res = 0;
+
+	argc = parse_options(argc, argv, NULL, options,
+			     usage, 0);
+
+	setup_enlistment_directory(argc, argv, usage, options, &zip_path);
+
+	strbuf_addstr(&zip_path, "/.scalarDiagnostics/scalar_");
+	strbuf_addftime(&zip_path,
+			"%Y%m%d_%H%M%S", localtime_r(&now, &tm), 0, 0);
+	strbuf_addstr(&zip_path, ".zip");
+	switch (safe_create_leading_directories(zip_path.buf)) {
+	case SCLD_EXISTS:
+	case SCLD_OK:
+		break;
+	default:
+		error_errno(_("could not create directory for '%s'"),
+			    zip_path.buf);
+		goto diagnose_cleanup;
+	}
+	stdout_fd = dup(1);
+	if (stdout_fd < 0) {
+		res = error_errno(_("could not duplicate stdout"));
+		goto diagnose_cleanup;
+	}
+
+	archiver_fd = xopen(zip_path.buf, O_CREAT | O_WRONLY | O_TRUNC, 0666);
+	if (archiver_fd < 0 || dup2(archiver_fd, 1) < 0) {
+		res = error_errno(_("could not redirect output"));
+		goto diagnose_cleanup;
+	}
+
+	init_zip_archiver();
+	strvec_pushl(&archiver_args, "scalar-diagnose", "--format=zip", NULL);
+
+	strbuf_reset(&buf);
+	strbuf_addstr(&buf, "Collecting diagnostic info\n\n");
+	get_version_info(&buf, 1);
+
+	strbuf_addf(&buf, "Enlistment root: %s\n", the_repository->worktree);
+	get_disk_info(&buf);
+	write_or_die(stdout_fd, buf.buf, buf.len);
+	strvec_pushf(&archiver_args,
+		     "--add-virtual-file=diagnostics.log:%.*s",
+		     (int)buf.len, buf.buf);
+
+	strbuf_reset(&buf);
+	strbuf_addstr(&buf, "--add-virtual-file=packs-local.txt:");
+	dir_file_stats(the_repository->objects->odb, &buf);
+	foreach_alt_odb(dir_file_stats, &buf);
+	strvec_push(&archiver_args, buf.buf);
+
+	strbuf_reset(&buf);
+	strbuf_addstr(&buf, "--add-virtual-file=objects-local.txt:");
+	loose_objs_stats(&buf, ".git/objects");
+	strvec_push(&archiver_args, buf.buf);
+
+	if ((res = add_directory_to_archiver(&archiver_args, ".git", 0)) ||
+	    (res = add_directory_to_archiver(&archiver_args, ".git/hooks", 0)) ||
+	    (res = add_directory_to_archiver(&archiver_args, ".git/info", 0)) ||
+	    (res = add_directory_to_archiver(&archiver_args, ".git/logs", 1)) ||
+	    (res = add_directory_to_archiver(&archiver_args, ".git/objects/info", 0)))
+		goto diagnose_cleanup;
+
+	strvec_pushl(&archiver_args, "--prefix=",
+		     oid_to_hex(the_hash_algo->empty_tree), "--", NULL);
+
+	/* `write_archive()` modifies the `argv` passed to it. Let it. */
+	argv_copy = xmemdupz(archiver_args.v,
+			     sizeof(char *) * archiver_args.nr);
+	res = write_archive(archiver_args.nr, (const char **)argv_copy, NULL,
+			    the_repository, NULL, 0);
+	if (res) {
+		error(_("failed to write archive"));
+		goto diagnose_cleanup;
+	}
+
+	if (!res)
+		fprintf(stderr, "\n"
+		       "Diagnostics complete.\n"
+		       "All of the gathered info is captured in '%s'\n",
+		       zip_path.buf);
+
+diagnose_cleanup:
+	if (archiver_fd >= 0) {
+		close(1);
+		dup2(stdout_fd, 1);
+	}
+	free(argv_copy);
+	strvec_clear(&archiver_args);
+	strbuf_release(&zip_path);
+	strbuf_release(&path);
+	strbuf_release(&buf);
+
+	return res;
+}
+
 static int cmd_list(int argc, const char **argv)
 {
 	if (argc != 1)
@@ -799,6 +1086,7 @@ static struct {
 	{ "reconfigure", cmd_reconfigure },
 	{ "delete", cmd_delete },
 	{ "version", cmd_version },
+	{ "diagnose", cmd_diagnose },
 	{ NULL, NULL},
 };
 
diff --git a/contrib/scalar/scalar.txt b/contrib/scalar/scalar.txt
index cf4e5b889c..c0425e0653 100644
--- a/contrib/scalar/scalar.txt
+++ b/contrib/scalar/scalar.txt
@@ -14,6 +14,7 @@ scalar register [<enlistment>]
 scalar unregister [<enlistment>]
 scalar run ( all | config | commit-graph | fetch | loose-objects | pack-files ) [<enlistment>]
 scalar reconfigure [ --all | <enlistment> ]
+scalar diagnose [<enlistment>]
 scalar delete <enlistment>
 
 DESCRIPTION
@@ -139,6 +140,17 @@ reconfigure the enlistment.
 With the `--all` option, all enlistments currently registered with Scalar
 will be reconfigured. Use this option after each Scalar upgrade.
 
+Diagnose
+~~~~~~~~
+
+diagnose [<enlistment>]::
+    When reporting issues with Scalar, it is often helpful to provide the
+    information gathered by this command, including logs and certain
+    statistics describing the data shape of the current enlistment.
++
+The output of this command is a `.zip` file that is written into
+a directory adjacent to the worktree in the `src` directory.
+
 Delete
 ~~~~~~
 
diff --git a/contrib/scalar/t/t9099-scalar.sh b/contrib/scalar/t/t9099-scalar.sh
index 89781568f4..10b1172a8a 100755
--- a/contrib/scalar/t/t9099-scalar.sh
+++ b/contrib/scalar/t/t9099-scalar.sh
@@ -93,4 +93,31 @@ test_expect_success 'scalar supports -c/-C' '
 	test true = "$(git -C sub config core.preloadIndex)"
 '
 
+test_expect_success '`scalar [...] <dir>` errors out when dir is missing' '
+	! scalar run config cloned 2>err &&
+	grep "cloned. does not exist" err
+'
+
+SQ="'"
+test_expect_success UNZIP 'scalar diagnose' '
+	scalar clone "file://$(pwd)" cloned --single-branch &&
+	git repack &&
+	echo "$(pwd)/.git/objects/" >>cloned/src/.git/objects/info/alternates &&
+	test_commit -C cloned/src loose &&
+	scalar diagnose cloned >out 2>err &&
+	grep "Available space" out &&
+	sed -n "s/.*$SQ\\(.*\\.zip\\)$SQ.*/\\1/p" <err >zip_path &&
+	zip_path=$(cat zip_path) &&
+	test -n "$zip_path" &&
+	unzip -v "$zip_path" &&
+	folder=${zip_path%.zip} &&
+	test_path_is_missing "$folder" &&
+	unzip -p "$zip_path" diagnostics.log >out &&
+	test_file_not_empty out &&
+	unzip -p "$zip_path" packs-local.txt >out &&
+	grep "$(pwd)/.git/objects" out &&
+	unzip -p "$zip_path" objects-local.txt >out &&
+	grep "^Total: [1-9]" out
+'
+
 test_done
diff --git a/t/t5003-archive-zip.sh b/t/t5003-archive-zip.sh
index d726964307..3992d08158 100755
--- a/t/t5003-archive-zip.sh
+++ b/t/t5003-archive-zip.sh
@@ -206,6 +206,26 @@ test_expect_success 'git archive --format=zip --add-file' '
 check_zip with_untracked
 check_added with_untracked untracked untracked
 
+test_expect_success UNZIP 'git archive --format=zip --add-virtual-file' '
+	if test_have_prereq FUNNYNAMES
+	then
+		PATHNAME="pathname with : colon"
+	else
+		PATHNAME="pathname without colon"
+	fi &&
+	git archive --format=zip >with_file_with_content.zip \
+		--add-virtual-file=\""$PATHNAME"\": \
+		--add-virtual-file=hello:world $EMPTY_TREE &&
+	test_when_finished "rm -rf tmp-unpack" &&
+	mkdir tmp-unpack && (
+		cd tmp-unpack &&
+		"$GIT_UNZIP" ../with_file_with_content.zip &&
+		test_path_is_file hello &&
+		test_path_is_file "$PATHNAME" &&
+		test world = $(cat hello)
+	)
+'
+
 test_expect_success 'git archive --format=zip --add-file twice' '
 	echo untracked >untracked &&
 	git archive --format=zip --prefix=one/ --add-file=untracked \