stacktrace_build() allocates num_mods module path pairs, one per frame location with line information, and returns the raw stacktrace when malloc fails. When no frame has line information, num_mods is 0, and on ESP-IDF malloc(0) returns NULL (heap_caps_base.c), so the stacktrace comes back raw ({NumFrames, 0, 0, 0, [{ModIndex, Offset}, ...], Class}) instead of a list, and stderr shows:

Unable to allocate space for module paths.  Returning raw stacktrace.

That covers every application packed without line information on ESP32: we saw it on all our boards (ESP32 and ESP32-S3 with 8 MB of PSRAM), never related to memory. glibc returns a non-NULL pointer for malloc(0), which is why generic_unix never shows it.

The fix allocates only when num_mods > 0. With 0 the loop never touches module_paths, and free(NULL) is fine.

Test. test_stacktrace (ESP32) raises in a module compiled with no_line_info and checks that the stacktrace is a list with a frame of that module. The ESP32 test project never declared AVM_CREATE_STACKTRACES, so stacktraces were undefined there; it now enables it as src/platforms/esp32/CMakeLists.txt does, which is what a device runs with.

• QEMU esp32, without the fix: the message above, and {not_a_list,{2,0,0,0,[{0,25},{0,20,erlang,error,1}],error}}; with it, test_stacktrace passes (test_socket fails here as it does on plain release-0.7, no network inside QEMU).
• generic_unix test-erlang: all pass.
• On hardware: one ESP32 with the fix printed the message 0 times in two days; boards without it, 109 and 13 times in the same period.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later