Drop hard dependency on elfutils 0.188

README.md

@@ -63,7 +63,9 @@ Check your package manager on how to install these dependencies (e.g.,
 `apt-get install libdw-dev libelf-dev` in Debian-based systems). Note that you may need to tell the
 compiler where to find the header and library files of the dependencies for the build to
 succeed. Check your distribution's documentation to determine the location of the header and
-library files or for more detailed information.
+library files or for more detailed information. When building on Alpine Linux (or any other
+distribution that doesn't use glibc) you'll need elfutils 0.188 or newer. You may need to build this
+from source if your distribution's package manager doesn't have it.
 
 Once you have these binary dependencies installed, you can clone the repository and follow the
 typical build process for Python libraries:

src/pystack/_pystack/unwinder.cpp

@@ -85,20 +85,37 @@ frameCallback(Dwfl_Frame* state, void* arg)
         return -1;
     }
 
+    std::optional<Dwarf_Word> stackPointer;
+    // Unwinding through musl libc with elfutils can get stuck returning the
+    // same PC in a loop forever.
+    //
+    //   https://sourceware.org/bugzilla/show_bug.cgi?id=30272
+    //   https://marc.info/?l=musl&m=168053842303968&w=2
+    //
+    // We can work around this by asking elfutils what the stack pointer is for
+    // each frame and breaking out on our own if two different frames report
+    // the same stack pointer. As an optimization (and to avoid a hard
+    // dependency on a very recent version of elfutils), only do this check
+    // when PyStack isn't built against glibc. This isn't entirely correct, as
+    // it means that a PyStack built against glibc can fail to collect stacks
+    // for Python interpreters built against musl libc, but it's unlikely that
+    // users will encounter that. If they do the simple work around is to
+    // run PyStack using the same interpreter they want to get stacks for.
+
+#if _ELFUTILS_VERSION >= 188 or (defined(__linux__) && !defined(__GLIBC__))
+
     // These platform specific magic numbers are part of the platform ABI.
     // For any platform not handled below we never look up the value of the
     // stack pointer register, and so never return DWARF_CB_ABORT.
-    std::optional<Dwarf_Word> stackPointer;
     std::optional<unsigned int> stackPointerRegNo;
-
-#if defined(__linux__) && defined(__x86_64__)
+#    if defined(__x86_64__)
     // https://refspecs.linuxbase.org/elf/x86_64-abi-0.99.pdf
     // Figure 3.36: DWARF Register Number Mapping
     stackPointerRegNo = 7;
-#elif defined(__linux__) && defined(__aarch64__)
+#    elif defined(__aarch64__)
     // https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#DW-REG
     stackPointerRegNo = 31;
-#endif
+#    endif
 
     if (stackPointerRegNo) {
         stackPointer.emplace(0);
@@ -113,6 +130,7 @@ frameCallback(Dwfl_Frame* state, void* arg)
         LOG(DEBUG) << std::hex << std::showbase << "Breaking out of (infinite?) unwind loop @ " << pc;
         return DWARF_CB_ABORT;
     }
+#endif
 
     frames->emplace_back(pc, isActivation, stackPointer);
     return DWARF_CB_OK;