IdleWakeups: skip processes that report 0 threads

This change fixes IdleWakeups' SystemInformationSampler failing to
handle processes for which ::NtQuerySystemInformation() returns a
thread count of zero. With this change, processes with "zero" threads
are simply skipped.

Currently, the "Secure System" process reports a thread count of 0
(as it prohibits other processes from accessing it in any way), and
SystemInformationSampler subtracts 1 from this (logically assuming
that every process has at least one thread), which overflows to a
large number. SystemInformationSampler, noting that thread
information for <large number> threads would not fit in its buffer,
triggers an early return.

In practice, this results in IdleWakeups showing 0 processes when
a process with "zero" threads exists before its targeted process in
the process list, because it hits the early return described above
before reaching the process of interest.

Change-Id: I63b9a36bb139aba0b18539e65312bdc5d836cc69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335799
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795542}

tools/win/IdleWakeups/system_information_sampler.cpp

@@ -283,6 +283,9 @@ std::unique_ptr<ProcessDataSnapshot> SystemInformationSampler::TakeSnapshot() {
     auto pi = reinterpret_cast<const SYSTEM_PROCESS_INFORMATION*>(
         data_buffer.data() + offset);
 
+    // Skip processes that report zero threads (e.g., the "Secure System"
+    // process, which does not disclose its thread count).
+    if (pi->NumberOfThreads > 0) {
       // Validate that |pi| and any additional SYSTEM_THREAD_INFORMATION structs
       // that it may have are all within the buffer boundary.
       if (offset + sizeof(SYSTEM_PROCESS_INFORMATION) +
@@ -306,6 +309,7 @@ std::unique_ptr<ProcessDataSnapshot> SystemInformationSampler::TakeSnapshot() {
               std::make_pair(pi->ProcessId, GetProcessData(pi)));
         }
       }
+    }
 
     // Check for end of the list.
     if (!pi->NextEntryOffset)