Don't trap into the debugger on panics under Linux

This breaks `rr`, see rust-lang#129019 (comment)
for the discussion

CC @khuey @workingjubilee

Author: kromych

std/src/sys/dbg.rs

@@ -105,84 +105,7 @@ mod os {
     }
 }
 
-#[cfg(target_os = "linux")]
-mod os {
-    use super::DebuggerPresence;
-    use crate::fs::File;
-    use crate::io::Read;
-
-    pub(super) fn is_debugger_present() -> Option<DebuggerPresence> {
-        // This function is crafted with the following goals:
-        // * Memory efficiency: It avoids crashing the panicking process due to
-        //   out-of-memory (OOM) conditions by not using large heap buffers or
-        //   allocating significant stack space, which could lead to stack overflow.
-        // * Minimal binary size: The function uses a minimal set of facilities
-        //   from the standard library to avoid increasing the resulting binary size.
-        //
-        // To achieve these goals, the function does not use `[std::io::BufReader]`
-        // and instead reads the file byte by byte using a sliding window approach.
-        // It's important to note that the "/proc/self/status" pseudo-file is synthesized
-        // by the Virtual File System (VFS), meaning it is not read from a slow or
-        // non-volatile storage medium so buffering might not be as beneficial because
-        // all data is read from memory, though this approach does incur a syscall for
-        // each byte read.
-        //
-        // We cannot make assumptions about the file size or the position of the
-        // target prefix ("TracerPid:"), so the function does not use
-        // `[std::fs::read_to_string]` thus not employing UTF-8 to ASCII checking,
-        // conversion, or parsing as we're looking for an ASCII prefix.
-        //
-        // These condiderations make the function deviate from the familiar concise pattern
-        // of searching for a string in a text file.
-
-        fn read_byte(file: &mut File) -> Option<u8> {
-            let mut buffer = [0];
-            file.read_exact(&mut buffer).ok()?;
-            Some(buffer[0])
-        }
-
-        // The ASCII prefix of the datum we're interested in.
-        const TRACER_PID: &[u8] = b"TracerPid:\t";
-
-        let mut file = File::open("/proc/self/status").ok()?;
-        let mut matched = 0;
-
-        // Look for the `TRACER_PID` prefix.
-        while let Some(byte) = read_byte(&mut file) {
-            if byte == TRACER_PID[matched] {
-                matched += 1;
-                if matched == TRACER_PID.len() {
-                    break;
-                }
-            } else {
-                matched = 0;
-            }
-        }
-
-        // Was the prefix found?
-        if matched != TRACER_PID.len() {
-            return None;
-        }
-
-        // It was; get the ASCII representation of the first digit
-        // of the PID. That is enough to see if there is a debugger
-        // attached as the kernel does not pad the PID on the left
-        // with the leading zeroes.
-        let byte = read_byte(&mut file)?;
-        if byte.is_ascii_digit() && byte != b'0' {
-            Some(DebuggerPresence::Detected)
-        } else {
-            Some(DebuggerPresence::NotDetected)
-        }
-    }
-}
-
-#[cfg(not(any(
-    target_os = "windows",
-    target_vendor = "apple",
-    target_os = "freebsd",
-    target_os = "linux"
-)))]
+#[cfg(not(any(target_os = "windows", target_vendor = "apple", target_os = "freebsd")))]
 mod os {
     pub(super) fn is_debugger_present() -> Option<super::DebuggerPresence> {
         None