std: rework/remove ucontext_t

Our usage of `ucontext_t` in the standard library was kind of
problematic. We unnecessarily mimiced libc-specific structures, and our
`getcontext` implementation was overkill for our use case of stack
tracing.

This commit introduces a new namespace, `std.debug.cpu_context`, which
contains "context" types for various architectures (currently x86,
x86_64, ARM, and AARCH64) containing the general-purpose CPU registers;
the ones needed in practice for stack unwinding. Each implementation has
a function `current` which populates the structure using inline
assembly. The structure is user-overrideable, though that should only be
necessary if the standard library does not have an implementation for
the *architecture*: that is to say, none of this is OS-dependent.

Of course, in POSIX signal handlers, we get a `ucontext_t` from the
kernel. The function `std.debug.cpu_context.fromPosixSignalContext`
converts this to a `std.debug.cpu_context.Native` with a big ol' target
switch.

This functionality is not exposed from `std.c` or `std.posix`, and
neither are `ucontext_t`, `mcontext_t`, or `getcontext`. The rationale
is that these types and functions do not conform to a specific ABI, and
in fact tend to get updated over time based on CPU features and
extensions; in addition, different libcs use different structures which
are "partially compatible" with the kernel structure. Overall, it's a
mess, but all we need is the kernel context, so we can just define a
kernel-compatible structure as long as we don't claim C compatibility by
putting it in `std.c` or `std.posix`.

This change resulted in a few nice `std.debug` simplifications, but
nothing too noteworthy. However, the main benefit of this change is that
DWARF unwinding---sometimes necessary for collecting stack traces
reliably---now requires far less target-specific integration.

Also fix a bug I noticed in `PageAllocator` (I found this due to a bug
in my distro's QEMU distribution; thanks, broken QEMU patch!) and I
think a couple of minor bugs in `std.debug`.

Resolves: #23801
Resolves: #23802

1 files changed, 1 insertions, 2 deletions

diff --git a/lib/std/debug/SelfInfo/ElfModule.zig b/lib/std/debug/SelfInfo/ElfModule.zig
index 8a0acf8bb0..fde61d8140 100644
--- a/lib/std/debug/SelfInfo/ElfModule.zig
+++ b/lib/std/debug/SelfInfo/ElfModule.zig
@@ -26,7 +26,6 @@ pub fn key(m: ElfModule) usize {
 pub fn lookup(cache: *LookupCache, gpa: Allocator, address: usize) Error!ElfModule {
     _ = cache;
     _ = gpa;
-    if (builtin.target.os.tag == .haiku) @panic("TODO implement lookup module for Haiku");
     const DlIterContext = struct {
         /// input
         address: usize,
@@ -261,7 +260,7 @@ pub const supports_unwinding: bool = s: {
 };
 comptime {
     if (supports_unwinding) {
-        std.debug.assert(Dwarf.abi.supportsUnwinding(&builtin.target));
+        std.debug.assert(Dwarf.supportsUnwinding(&builtin.target));
     }
 }