pillow-heif's sdist build failed with RequiredDependencyException: the
container toolchain installs pillow's libs but not libheif, so
pkg-config libheif had nothing to answer with. Termux's libheif 1.23.4
ships the headers and libheif.pc directly (no -dev split).
Runtime: the built _pillow_heif extension DT_NEEDEDs libheif.so, which
pulls libx265/libde265/libaom/libx264/libc++_shared. All but libheif
and libde265 were already staged in runtime_libs.json (verified by
reading the .so's DT_NEEDED, not the deb Depends line, which also
lists gdk-pixbuf/glib/rav1e the linker never loads); pin those two.
Import gate: import pillow_heif never fails on a dead link -- its
__init__ swallows the _pillow_heif ImportError into a DeferredError
that fires only on first use. The gate now imports _pillow_heif
directly so a dlopen failure fails the build instead of the phone.
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