Upgrade to ESP-IDF v6.0 (picolibc replaces newlib as default)

[ltowarek]

ESP-IDF v6.0 (latest: v6.0.1, released April 2026) is now available and introduces a significant change for this project: picolibc replaces newlib as the default C library for GCC toolchains.

The switch is controlled by a new Kconfig in components/esp_libc:

choice LIBC
    default LIBC_NEWLIB if IDF_TOOLCHAIN_CLANG
    default LIBC_PICOLIBC

This directly affects several open workaround issues that exist because of newlib gaps:

• newlib / ESP-IDF pthread: implement nanosleep() and pthread_atfork() #17 — nanosleep() and pthread_atfork() stubs
• newlib: sysconf(_SC_PAGESIZE) returns -1 instead of a real page size #18 — sysconf(_SC_PAGESIZE) linker wrap
• newlib: ship <sys/mman.h> #19 — <sys/mman.h> shim
• newlib: extend struct tm with tm_gmtoff and tm_zone #20 — struct tm extension shim

Each of those workarounds should be re-evaluated against picolibc — some may be removable entirely.

Tasks

• Test the component against ESP-IDF v6.0 (start with v6.0.1)
• Audit which workarounds in newlib / ESP-IDF pthread: implement nanosleep() and pthread_atfork() #17–newlib: extend struct tm with tm_gmtoff and tm_zone #20 are no longer needed under picolibc
• Update idf_component.yml minimum version requirement if needed
• Check for any breaking API or CMake changes in v6.0 that affect the integration
• Update CI to use espressif/idf:release-v6.0

[ltowarek]

Update: PR #52 implements the v6 upgrade.

Progress against the checklist:

• Test the component against ESP-IDF v6.0 — PR Upgrade ESP-IDF to v6 #52 contains all v6-specific workarounds
• Audit workarounds newlib / ESP-IDF pthread: implement nanosleep() and pthread_atfork() #17–newlib: extend struct tm with tm_gmtoff and tm_zone #20 under picolibc — all four remain required (picolibc on ESP-IDF v6 does not provide nanosleep, pthread_atfork, sysconf(_SC_PAGESIZE), sys/mman.h, or struct tm extensions)
• Update idf_component.yml minimum version — bumped to >=6.0.0 (v6 linker symbols in posix_shims.c are incompatible with v5)
• Check for breaking API/CMake changes in v6.0 — addressed: THREADPTR linker symbols renamed (see ESP-IDF startup: initialize Xtensa THREADPTR before C++ global constructors #21), --orphan-handling=error requires ABSL_ATTRIBUTE_SECTION suppression (see protobuf v34: descriptor tables marked __attribute__((section("protodesc_cold"))) #27), GCC 15 requires broader -Wno-error (see protobuf v34 arena: -Wmaybe-uninitialized false positives on Xtensa GCC 13.2 #29), and ESP-IDF v6 #if CONFIG_* pattern requires -Wno-undef (new issue)
• Update CI to use espressif/idf:release-v6.0 — .devcontainer/Dockerfile updated

New workaround introduced in PR #52: src/workarounds/otlp_http_exporter_esp.cc — fixes the protobuf Arena alignment crash (see #32).