Allow getpid in isolation mode, add gettid support

In order to support gettid when isolation is enabled and when it is
disabled, and satisfy its requirement that:

   In a single-threaded process, the thread ID is equal to the process ID
   (PID, as returned by getpid(2)).

we define the thread ID to be getpid() + <miri internal thread id>.
Since the internal thread id of the main thread is zero, this will
satisfy that requirement.

However, getpid for now was only supported when isolation was disabled.
To support getpid in isolation mode, we return a hardcoded value (1000)
and return that instead of the real PID.

Author: Mandragorian

src/shims/env.rs

@@ -108,4 +108,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             EnvVars::Windows(vars) => vars.get(name),
         }
     }
+
+    fn get_pid(&self) -> u32 {
+        let this = self.eval_context_ref();
+        if this.machine.communicate() { std::process::id() } else { 1000 }
+    }
 }

src/shims/unix/env.rs

@@ -274,12 +274,23 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let this = self.eval_context_mut();
         this.assert_target_os_is_unix("getpid");
 
-        this.check_no_isolation("`getpid`")?;
-
         // The reason we need to do this wacky of a conversion is because
         // `libc::getpid` returns an i32, however, `std::process::id()` return an u32.
         // So we un-do the conversion that stdlib does and turn it back into an i32.
         #[allow(clippy::cast_possible_wrap)]
-        Ok(std::process::id() as i32)
+        Ok(this.get_pid() as i32)
+    }
+
+    fn linux_gettid(&mut self) -> InterpResult<'tcx, i32> {
+        let this = self.eval_context_ref();
+        this.assert_target_os("linux", "gettid");
+
+        let index = this.machine.threads.active_thread().to_u32();
+
+        // Compute a TID for this thread, ensuring that the main thread has PID == TID.
+        let tid = this.get_pid().strict_add(index);
+
+        #[allow(clippy::cast_possible_wrap)]
+        Ok(tid as i32)
     }
 }

src/shims/unix/linux/foreign_items.rs

@@ -94,6 +94,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 )?;
                 this.write_scalar(res, dest)?;
             }
+            "gettid" => {
+                let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
+                let result = this.linux_gettid()?;
+                this.write_scalar(Scalar::from_i32(result), dest)?;
+            }
 
             // Dynamically invoked syscalls
             "syscall" => {

src/shims/windows/env.rs

@@ -200,9 +200,8 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn GetCurrentProcessId(&mut self) -> InterpResult<'tcx, u32> {
         let this = self.eval_context_mut();
         this.assert_target_os("windows", "GetCurrentProcessId");
-        this.check_no_isolation("`GetCurrentProcessId`")?;
 
-        Ok(std::process::id())
+        Ok(this.get_pid())
     }
 
     #[allow(non_snake_case)]

tests/pass-dep/libc/gettid.rs

@@ -0,0 +1,22 @@
+//@only-target-linux
+//@revisions: with_isolation without_isolation
+//@[without_isolation] compile-flags: -Zmiri-disable-isolation
+
+use libc::{getpid, gettid};
+use std::thread;
+
+fn main() {
+    thread::spawn(|| {
+        // Test that in isolation mode a deterministic value will be returned.
+        // The value 1001 is not important, we only care that whatever the value
+        // is, won't change from execution to execution.
+        #[cfg(with_isolation)]
+        assert_eq!(unsafe { gettid() }, 1001);
+
+        assert_ne!(unsafe { gettid() }, unsafe { getpid() });
+    });
+
+    // Test that the thread ID of the main thread is the same as the process
+    // ID.
+    assert_eq!(unsafe { gettid() }, unsafe { getpid() });
+}

tests/pass/getpid.rs

@@ -1,9 +1,20 @@
-//@compile-flags: -Zmiri-disable-isolation
+//@revisions: with_isolation without_isolation
+//@[without_isolation] compile-flags: -Zmiri-disable-isolation
 
 fn getpid() -> u32 {
     std::process::id()
 }
 
 fn main() {
-    getpid();
+    let pid = getpid();
+
+    std::thread::spawn(move || {
+        assert_eq!(getpid(), pid);
+    });
+
+    // Test that in isolation mode a deterministic value will be returned.
+    // The value 1000 is not important, we only care that whatever the value
+    // is, won't change from execution to execution.
+    #[cfg(with_isolation)]
+    assert_eq!(pid, 1000);
 }