[libc][signal] clean up usage of sighandler_t (llvm#125745)

`man 3 signal`'s declaration has a face _only a mother could love_.

sighandler_t and __sighandler_t are not defined in the C standard, or POSIX.

They are helpful typedefs provided by glibc and the Linux kernel UAPI headers
respectively since working with function pointers' syntax can be painful. But
we should not rely on them; in C++ we have `auto*` and `using` statements.

Remove the proxy header, and only include a typedef for sighandler_t when
targeting Linux, for compatibility with glibc.

Fixes: llvm#125598

Author: nickdesaulniers

libc/hdr/types/CMakeLists.txt

@@ -250,15 +250,6 @@ add_proxy_header_library(
     libc.include.locale
 )
 
-add_proxy_header_library(
-  sighandler_t
-  HDRS
-    sighandler_t.h
-  FULL_BUILD_DEPENDS
-    libc.include.llvm-libc-types.__sighandler_t
-    libc.include.signal
-)
-
 add_proxy_header_library(
   stack_t
   HDRS

libc/hdr/types/sighandler_t.h

@@ -284,13 +284,14 @@ add_header_macro(
   signal.h
   DEPENDS
     .llvm-libc-macros.signal_macros
+    .llvm-libc-types.pid_t
     .llvm-libc-types.sig_atomic_t
+    .llvm-libc-types.sighandler_t
+    .llvm-libc-types.siginfo_t
     .llvm-libc-types.sigset_t
+    .llvm-libc-types.stack_t
     .llvm-libc-types.struct_sigaction
     .llvm-libc-types.union_sigval
-    .llvm-libc-types.siginfo_t
-    .llvm-libc-types.stack_t
-    .llvm-libc-types.pid_t
 )
 
 add_header_macro(

libc/include/CMakeLists.txt

@@ -16,9 +16,9 @@
 #define SIGSEGV 11
 #define SIGTERM 15
 
-#define SIG_DFL ((__sighandler_t)(0))
-#define SIG_IGN ((__sighandler_t)(1))
-#define SIG_ERR ((__sighandler_t)(-1))
+#define SIG_DFL ((void (*)(int))(0))
+#define SIG_IGN ((void (*)(int))(1))
+#define SIG_ERR ((void (*)(int))(-1))
 
 // Max signal number
 #define NSIG 64

libc/include/llvm-libc-macros/gpu/signal-macros.h

@@ -86,9 +86,9 @@
 #error "Signal stack sizes not defined for your platform."
 #endif
 
-#define SIG_DFL ((__sighandler_t)0)
-#define SIG_IGN ((__sighandler_t)1)
-#define SIG_ERR ((__sighandler_t)-1)
+#define SIG_DFL ((void (*)(int))0)
+#define SIG_IGN ((void (*)(int))1)
+#define SIG_ERR ((void (*)(int))(-1))
 
 // SIGCHLD si_codes
 #define CLD_EXITED 1    // child has exited

libc/include/llvm-libc-macros/linux/signal-macros.h

@@ -15,7 +15,6 @@ add_header(__pthread_start_t HDR __pthread_start_t.h)
 add_header(__pthread_tss_dtor_t HDR __pthread_tss_dtor_t.h)
 add_header(__qsortcompare_t HDR __qsortcompare_t.h)
 add_header(__qsortrcompare_t HDR __qsortrcompare_t.h)
-add_header(__sighandler_t HDR __sighandler_t.h)
 add_header(__thread_type HDR __thread_type.h)
 add_header(blkcnt_t HDR blkcnt_t.h)
 add_header(blksize_t HDR blksize_t.h)
@@ -66,6 +65,7 @@ if(LIBC_TYPES_TIME_T_IS_32_BIT)
 else()
   add_header(time_t HDR time_t_64.h DEST_HDR time_t.h)
 endif()
+add_header(sighandler_t HDR sighandler_t.h)
 add_header(stack_t HDR stack_t.h DEPENDS .size_t)
 add_header(suseconds_t HDR suseconds_t.h)
 add_header(struct_dirent HDR struct_dirent.h DEPENDS .ino_t .off_t)

libc/include/llvm-libc-types/CMakeLists.txt

@@ -1,14 +1,17 @@
-//===-- Definition of struct __sighandler_t -------------------------------===//
+//===-- Definition of sighandler_t ----------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_TYPES___SIGHANDLER_T_H
-#define LLVM_LIBC_TYPES___SIGHANDLER_T_H
+#ifndef LLVM_LIBC_TYPES_SIGHANDLER_T_H
+#define LLVM_LIBC_TYPES_SIGHANDLER_T_H
 
-typedef void (*__sighandler_t)(int);
+#ifdef __linux__
+// For compatibility with glibc.
+typedef void (*sighandler_t)(int);
+#endif
 
-#endif // LLVM_LIBC_TYPES___SIGHANDLER_T_H
+#endif // LLVM_LIBC_TYPES_SIGHANDLER_T_H

libc/include/llvm-libc-types/__sighandler_t.h renamed to libc/include/llvm-libc-types/sighandler_t.h

@@ -25,6 +25,4 @@ struct sigaction {
 #endif
 };
 
-typedef void (*__sighandler_t)(int);
-
 #endif // LLVM_LIBC_TYPES_STRUCT_SIGACTION_H

libc/include/llvm-libc-types/struct_sigaction.h

@@ -3,12 +3,13 @@ header_template: signal.h.def
 macros: []
 types:
   - type_name: pid_t
-  - type_name: stack_t
+  - type_name: sig_atomic_t
+  - type_name: sighandler_t
   - type_name: siginfo_t
-  - type_name: struct_sigaction
   - type_name: sigset_t
+  - type_name: stack_t
+  - type_name: struct_sigaction
   - type_name: union_sigval
-  - type_name: sig_atomic_t
 enums: []
 objects: []
 functions:
@@ -69,10 +70,15 @@ functions:
   - name: signal
     standards:
       - stdc
-    return_type: __sighandler_t
+    # May the Geneva Convention have mercy on my soul... Why this insanity?
+    # Well: signal returns a function pointer to a function with no return
+    # value and which accepts an int. The parameter list appears on the far
+    # right of the declaration. i.e.
+    # void (*signal(int, void (*)(int)))(int);
+    return_type: void (*
     arguments:
       - type: int
-      - type: __sighandler_t
+      - type: void (*)(int)))(int
   - name: sigprocmask
     standards:
       - POSIX

libc/include/signal.yaml

@@ -127,7 +127,6 @@ add_entrypoint_object(
   DEPENDS
     .sigaction
     libc.hdr.signal_macros
-    libc.hdr.types.sighandler_t
 )
 
 add_entrypoint_object(