Fix-up behavior for empty PerfGroup

For machines that don't have `perf` properly configured (e.g. WSL),
it's common to end up with an empty PerfGroup that has an invalid
`leader_fd`. This commit adds the appropriate guards to make sure
that doesn't error excessively.

Author: topolarity

src/LinuxPerf.jl

@@ -369,6 +369,7 @@ const PERF_EVENT_IOC_DISABLE = UInt64(0x2401)
 const PERF_EVENT_IOC_RESET =   UInt64(0x2403)
 
 function ioctl(group::EventGroup, x)
+    group.leader_fd == -1 && return nothing
     res = ccall(:ioctl, Cint, (Cint, Clong, Clong), group.leader_fd, x, 1)
     Base.systemerror(:ioctl, res < 0)
     return nothing
@@ -383,6 +384,7 @@ function Base.close(g::EventGroup)
         fd == g.leader_fd && continue # close leader_fd last
         ccall(:close, Cint, (Cint,), fd)
     end
+    g.leader_fd == -1 && return
     ccall(:close, Cint, (Cint,), g.leader_fd)
 end
 
@@ -441,6 +443,7 @@ function counters(b::PerfBench)
     c = Counter[]
     for g in b.groups
         values = Vector{UInt64}(undef, length(g)+1+2)
+        g.leader_fd == -1 && continue # empty group
         read!(g.leader_io, values)
         #?Ref@assert(length(g) == values[1])
         enabled, running = values[2], values[3]
@@ -801,6 +804,10 @@ function ThreadStats(b::PerfBench)
     groups = Vector{Counter}[]
     for g in b.groups
         values = Vector{UInt64}(undef, length(g)+1+2)
+        if g.leader_fd == -1
+            push!(groups, Counter[]) # empty group
+            continue
+        end
         read!(g.leader_io, values)
         #?Ref@assert(length(g) == values[1])
         enabled, running = values[2], values[3]

test/runtests.jl

@@ -1,7 +1,7 @@
 using LinuxPerf
 using Test
 
-using LinuxPerf: make_bench, enable!, disable!, reset!, reasonable_defaults, counters, EventType, EventTypeExt, parse_groups, enable_all!, disable_all!
+using LinuxPerf: make_bench, enable!, disable!, reset!, reasonable_defaults, counters, EventGroup, EventType, EventTypeExt, parse_groups, enable_all!, disable_all!
 
 @testset "LinuxPerf" begin
 
@@ -128,4 +128,12 @@ end
     @test LinuxPerf._addcommas(typemin(Int64)) == "-9,223,372,036,854,775,808"
 end
 
+@testset "empty EventGroup" begin
+    # Creating an empty EventGroup should be a no-op (and not error)
+    g = EventGroup(EventType[])
+    @test length(g.fds) == 0
+    @test g.leader_fd == -1
+    close(g) # also a no-op
+end
+
 end