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, 0 insertions, 2 deletions

diff --git a/lib/std/os/linux.zig b/lib/std/os/linux.zig
index 48e9f08d5d..45cf597c76 100644
--- a/lib/std/os/linux.zig
+++ b/lib/std/os/linux.zig
@@ -49,7 +49,6 @@ const arch_bits = switch (native_arch) {
     .s390x => @import("linux/s390x.zig"),
     else => struct {
         pub const ucontext_t = void;
-        pub const getcontext = {};
     },
 };
 
@@ -112,7 +111,6 @@ pub const timeval = arch_bits.timeval;
 pub const timezone = arch_bits.timezone;
 pub const ucontext_t = arch_bits.ucontext_t;
 pub const user_desc = arch_bits.user_desc;
-pub const getcontext = arch_bits.getcontext;
 
 pub const tls = @import("linux/tls.zig");
 pub const BPF = @import("linux/bpf.zig");