stdlib: handle EEXIST in mmap with FIXED_NOREPLACE. Fixes #21475

Author: AlecFessler

lib/std/Thread.zig

@@ -1420,6 +1420,7 @@ const LinuxThreadImpl = struct {
             error.PermissionDenied => unreachable,
             error.ProcessFdQuotaExceeded => unreachable,
             error.SystemFdQuotaExceeded => unreachable,
+            error.MappingAlreadyExists => unreachable,
             else => |e| return e,
         };
         assert(mapped.len >= map_bytes);

lib/std/debug/Dwarf.zig

@@ -2434,14 +2434,17 @@ pub const ElfModule = struct {
         const end_pos = elf_file.getEndPos() catch return bad();
         const file_len = cast(usize, end_pos) orelse return error.Overflow;
 
-        const mapped_mem = try std.posix.mmap(
+        const mapped_mem = std.posix.mmap(
             null,
             file_len,
             std.posix.PROT.READ,
             .{ .TYPE = .SHARED },
             elf_file.handle,
             0,
-        );
+        ) catch |err| switch (err) {
+            error.MappingAlreadyExists => unreachable,
+            else => |e| return e,
+        };
         errdefer std.posix.munmap(mapped_mem);
 
         return load(

lib/std/posix.zig

@@ -4754,6 +4754,9 @@ pub const MMapError = error{
     ProcessFdQuotaExceeded,
     SystemFdQuotaExceeded,
     OutOfMemory,
+
+    /// Using FIXED_NOREPLACE flag and the process has already mapped memory at the given address
+    MappingAlreadyExists,
 } || UnexpectedError;
 
 /// Map files or devices into memory.
@@ -4792,6 +4795,7 @@ pub fn mmap(
         .MFILE => return error.ProcessFdQuotaExceeded,
         .NFILE => return error.SystemFdQuotaExceeded,
         .NOMEM => return error.OutOfMemory,
+        .EXIST => return error.MappingAlreadyExists,
         else => return unexpectedErrno(err),
     }
 }