More helpful error message for empty cpu_target in Base.julia_cmd (#52217)

Fix #52209.

Author: giordano

base/util.jl

@@ -144,7 +144,7 @@ See also [`print`](@ref), [`println`](@ref), [`show`](@ref).
     printstyled(stdout, msg...; bold=bold, italic=italic, underline=underline, blink=blink, reverse=reverse, hidden=hidden, color=color)
 
 """
-    Base.julia_cmd(juliapath=joinpath(Sys.BINDIR, julia_exename()); cpu_target)
+    Base.julia_cmd(juliapath=joinpath(Sys.BINDIR, julia_exename()); cpu_target::Union{Nothing,String}=nothing)
 
 Return a julia command similar to the one of the running process.
 Propagates any of the `--cpu-target`, `--sysimage`, `--compile`, `--sysimage-native-code`,
@@ -154,6 +154,8 @@ command line arguments that are not at their default values.
 
 Among others, `--math-mode`, `--warn-overwrite`, and `--trace-compile` are notably not propagated currently.
 
+Unless set to `nothing`, the `cpu_target` keyword argument can be used to override the CPU target set for the running process.
+
 To get the julia command without propagated command line arguments, `julia_cmd()[1]` can be used.
 
 !!! compat "Julia 1.1"
@@ -243,7 +245,7 @@ function julia_cmd(julia=joinpath(Sys.BINDIR, julia_exename()); cpu_target::Unio
     if opts.use_pkgimages == 0
         push!(addflags, "--pkgimages=no")
     end
-    return `$julia -C$cpu_target -J$image_file $addflags`
+    return `$julia -C $cpu_target -J$image_file $addflags`
 end
 
 function julia_exename()

test/cmdlineargs.jl

@@ -77,7 +77,9 @@ end
     opts = Base.JLOptions()
 
     for (arg, default) in (
-                            ("-C$(unsafe_string(opts.cpu_target))",  false),
+                            # Use a Cmd to handle space nicely when
+                            # interpolating inside another Cmd.
+                            (`-C $(unsafe_string(opts.cpu_target))`,  false),
 
                             ("-J$(unsafe_string(opts.image_file))",  false),
 
@@ -134,13 +136,21 @@ end
                             ("--pkgimages=no",  false),
                         )
         @testset "$arg" begin
+            str = arg isa Cmd ? join(arg.exec, ' ') : arg
             if default
-                @test !occursin(arg, get_julia_cmd(arg))
+                @test !occursin(str, get_julia_cmd(arg))
             else
-                @test occursin(arg, get_julia_cmd(arg))
+                @test occursin(str, get_julia_cmd(arg))
             end
         end
     end
+
+    # Test empty `cpu_target` gives a helpful error message, issue #52209.
+    io = IOBuffer()
+    p = run(pipeline(`$(Base.julia_cmd(; cpu_target="")) --startup-file=no -e ''`; stderr=io); wait=false)
+    wait(p)
+    @test p.exitcode == 1
+    @test occursin("empty CPU name", String(take!(io)))
 end
 
 let exename = `$(Base.julia_cmd()) --startup-file=no --color=no`