procinfo: defend against weird situations on Windows

I've had a couple of deadlock situations when invoking win32 apps
via wsl on my work machine.  In those situations, the process that
we show in the titlebar is a short lived process that has likely
recently terminated, so I suspect something weird about either
dangling references or, given the deadlock-y nature of things,
maybe trying to reference ourselves.

This commit tries to defend against these by:

* Avoiding creating a ProcHandle for our own pid
* Releasing the toolhelper snapshot as soon as we have copied
  out the list of pids that we need

Not sure that this will nail it, but it seems like it can't hurt.

procinfo/src/windows.rs

@@ -14,7 +14,7 @@ use winapi::shared::minwindef::{FILETIME, HMODULE, LPVOID, MAX_PATH};
 use winapi::shared::ntdef::{FALSE, NT_SUCCESS};
 use winapi::um::handleapi::CloseHandle;
 use winapi::um::memoryapi::ReadProcessMemory;
-use winapi::um::processthreadsapi::{GetProcessTimes, OpenProcess};
+use winapi::um::processthreadsapi::{GetCurrentProcessId, GetProcessTimes, OpenProcess};
 use winapi::um::psapi::{EnumProcessModulesEx, GetModuleFileNameExW, LIST_MODULES_ALL};
 use winapi::um::shellapi::CommandLineToArgvW;
 use winapi::um::tlhelp32::*;
@@ -40,6 +40,13 @@ impl Snapshot {
             first: true,
         }
     }
+
+    pub fn entries() -> Vec<PROCESSENTRY32W> {
+        match Self::new() {
+            Some(snapshot) => snapshot.iter().collect(),
+            None => vec![],
+        }
+    }
 }
 
 impl Drop for Snapshot {
@@ -92,16 +99,22 @@ struct ProcParams {
 }
 
 /// A handle to an opened process
-struct ProcHandle(HANDLE);
+struct ProcHandle {
+    proc: HANDLE,
+}
 
 impl ProcHandle {
     pub fn new(pid: u32) -> Option<Self> {
+        if pid == unsafe { GetCurrentProcessId() } {
+            // Avoid the potential for deadlock if we're examining ourselves
+            return None;
+        }
         let options = PROCESS_QUERY_INFORMATION | PROCESS_VM_READ;
         let handle = unsafe { OpenProcess(options, FALSE as _, pid) };
         if handle.is_null() {
             return None;
         }
-        Some(Self(handle))
+        Some(Self { proc: handle })
     }
 
     /// Returns the HMODULE for the process
@@ -111,7 +124,7 @@ impl ProcHandle {
         let size = std::mem::size_of_val(&hmod);
         let res = unsafe {
             EnumProcessModulesEx(
-                self.0,
+                self.proc,
                 hmod.as_mut_ptr(),
                 size as _,
                 &mut needed,
@@ -129,7 +142,8 @@ impl ProcHandle {
     pub fn executable(&self) -> Option<PathBuf> {
         let hmod = self.hmodule()?;
         let mut buf = [0u16; MAX_PATH + 1];
-        let res = unsafe { GetModuleFileNameExW(self.0, hmod, buf.as_mut_ptr(), buf.len() as _) };
+        let res =
+            unsafe { GetModuleFileNameExW(self.proc, hmod, buf.as_mut_ptr(), buf.len() as _) };
         if res == 0 {
             None
         } else {
@@ -142,7 +156,7 @@ impl ProcHandle {
         let mut data = MaybeUninit::<T>::uninit();
         let res = unsafe {
             NtQueryInformationProcess(
-                self.0,
+                self.proc,
                 what,
                 data.as_mut_ptr() as _,
                 std::mem::size_of::<T>() as _,
@@ -161,7 +175,7 @@ impl ProcHandle {
         let mut data = MaybeUninit::<T>::uninit();
         let res = unsafe {
             ReadProcessMemory(
-                self.0,
+                self.proc,
                 addr as _,
                 data.as_mut_ptr() as _,
                 std::mem::size_of::<T>() as _,
@@ -255,12 +269,18 @@ impl ProcHandle {
 
     /// Copies a sized WSTR from the address in the process
     fn read_process_wchar(&self, ptr: LPVOID, byte_size: usize) -> Option<Vec<u16>> {
+        if byte_size > MAX_PATH * 4 {
+            // Defend against implausibly large paths, just in
+            // case we're reading the wrong offset into a kernel struct
+            return None;
+        }
+
         let mut buf = vec![0u16; byte_size / 2];
         let mut bytes_read = 0;
 
         let res = unsafe {
             ReadProcessMemory(
-                self.0,
+                self.proc,
                 ptr as _,
                 buf.as_mut_ptr() as _,
                 byte_size,
@@ -305,7 +325,8 @@ impl ProcHandle {
         let mut kernel = empty();
         let mut user = empty();
 
-        let res = unsafe { GetProcessTimes(self.0, &mut start, &mut exit, &mut kernel, &mut user) };
+        let res =
+            unsafe { GetProcessTimes(self.proc, &mut start, &mut exit, &mut kernel, &mut user) };
         if res == 0 {
             return None;
         }
@@ -335,7 +356,7 @@ fn cmd_line_to_argv(buf: &[u16]) -> Vec<String> {
 
 impl Drop for ProcHandle {
     fn drop(&mut self) {
-        unsafe { CloseHandle(self.0) };
+        unsafe { CloseHandle(self.proc) };
     }
 }
 
@@ -352,8 +373,7 @@ impl LocalProcessInfo {
     }
 
     pub fn with_root_pid(pid: u32) -> Option<Self> {
-        let snapshot = Snapshot::new()?;
-        let procs: Vec<_> = snapshot.iter().collect();
+        let procs = Snapshot::entries();
 
         fn build_proc(info: &PROCESSENTRY32W, procs: &[PROCESSENTRY32W]) -> LocalProcessInfo {
             let mut children = HashMap::new();