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@select macro fixes #13853

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@select macro fixes #13853

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fadd3dd
select macro fixes
durcan Nov 3, 2015
512a3ef
@select macro doc and test update
durcan Nov 3, 2015
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146 changes: 76 additions & 70 deletions base/channels.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -201,18 +201,13 @@ A select expression of the form:

```julia
@select begin
if clause
body
elseif clause
body
[else
default_body
]
clause1 => body1
clause2 => body2
_ => default_body
end
end
```

Wait for multiple clauses simultaneously using an if-else syntax, taking a different action depending on which clause is available first.
Wait for multiple clauses simultaneously using a pattern matching syntax, taking a different action depending on which clause is available first.

A clause has three possible forms:

Expand Down Expand Up @@ -241,14 +236,12 @@ channel2 = Channel()
task = @task ...

result = @select begin
if channel1 |> value
info("Took from channel1")
value
elseif channel2 <| :test
info("Put :test into channel2")
elseif task
info("task finished")
end
channel1 |> value => begin
info("Took from channel1")
value
end
channel2 <| :test => info("Put :test into channel2")
task => info("task finished")
end
```
"""
Expand All @@ -258,25 +251,23 @@ macro select(expr)
# an 'else' clause is present in the @select body (in which case it will be
# nonblocking).
mode = :blocking
while true
# Be robust to extraneous blocks caused by whitespace by skipping over them
while isa(expr, Expr) && expr.head == :block
expr = expr.args[2] # args[1] is a LineNumber node
end
if isa(expr, Expr) && expr.head == :if
push!(clauses, (parse_select_clause(expr.args[1]), expr.args[2]))
if length(expr.args) == 3
expr = expr.args[3] # The 'elseif' block
for se in expr.args
# skip line nodes
isa(se, Expr) || continue
# grab all the pairs
if se.head == :(=>)
if se.args[1] != :_
push!(clauses, (parse_select_clause(se.args[1]), se.args[2]))
else
break
# The defaule case (_). If present, the select
# statement is considered non-blocking and will return this
# section if none of the other conditions are immediately available.
push!(clauses, (SelectClause(SelectDefault, Nullable(), Nullable()), se.args[2]))
mode = :nonblocking
end
else
# The 'else' section of an if statement. If present, the select
# statement is considered non-blocking and will return this
# section if none of the other conditions are immediately available.
push!(clauses, (SelectClause(SelectDefault, Nullable(), Nullable()), expr))
mode = :nonblocking
break
elseif se.head != :block && se.head != :line
# if we run into an expression that is not a block. line or pair throw an error
throw(ErrorException("Selection expressions must be Pairs. Found: $(se.head)"))
end
end
if mode == :nonblocking
Expand All @@ -302,42 +293,48 @@ _take!(x) = wait(x)
_isready(c::AbstractChannel) = isready(c)
_isready(t::Task) = istaskdone(t)

# helper function to place the default case in the proper position
function set_default_first!(clauses)
default_pos = find(clauses) do x
clause, body = x
clause.kind == SelectDefault
end
l = length(default_pos)
l == 0 && return # bail out if there is no default case
if l > 1
d = [clauses[i] for i in default_pos]
err_str = reduce(* , ["\n _ => $(string(b))" for (c, b) in d])
throw(ErrorException("Select takes at most one default case. Found: $l $err_str"))
end
# swap elements to sure make SelectDefault comes first
clauses[1], clauses[default_pos[1]] = clauses[default_pos[1]], clauses[1]
clauses
end

function _select_nonblock_macro(clauses)
set_default_first!(clauses)
branches = Expr(:block)
for (clause, body) in clauses
local branch
branch =
if clause.kind == SelectPut
branch = quote
if isready_put($(clause.channel|>get|>esc))
put!($(clause.channel|>get|>esc), $(clause.value|>get|>esc))
ret = $(esc(body))
break
end
end
:(if isready_put($(clause.channel|>get|>esc))
put!($(clause.channel|>get|>esc), $(clause.value|>get|>esc))
$(esc(body))
end)
elseif clause.kind == SelectTake
branch = quote
if _isready($(clause.channel|>get|>esc))
$(clause.value|>get|>esc) =
_take!($(clause.channel|>get|>esc))
ret = $(esc(body))
break
end
end
:(if _isready($(clause.channel|>get|>esc))
$(clause.value|>get|>esc) = _take!($(clause.channel|>get|>esc))
$(esc(body))
end)
elseif clause.kind == SelectDefault
branch = quote
ret = $(esc(body))
break
end
:($(esc(body)))
end
push!(branches.args, branch)
end
quote
local ret
while true
$branches
end
ret

# the next two lines build an if / elseif chain from the bottom up
push!(branch.args, branches)
branches = branch
end
:($branches)
end

# The strategy for blocking select statements is to create a set of "rival"
Expand Down Expand Up @@ -370,13 +367,16 @@ end

function _select_block_macro(clauses)
branches = Expr(:block)
body_branches = Expr(:block)
for (i, (clause, body)) in enumerate(clauses)
if clause.kind == SelectPut
wait_for_channel = :(wait_put($(clause.channel|>get|>esc)))
mutate_channel = :(put!($(clause.channel|>get|>esc), $(clause.value|>get|>esc)))
bind_variable = :(nothing)
elseif clause.kind == SelectTake
wait_for_channel = :(wait($(clause.channel|>get|>esc)))
mutate_channel = :($(clause.value|>get|>esc) = _take!($(clause.channel|>get|>esc)))
mutate_channel = :(_take!($(clause.channel|>get|>esc)))
bind_variable = :($(clause.value|>get|>esc) = branch_val)
end
branch = quote
tasks[$i] = @schedule begin
Expand All @@ -394,21 +394,27 @@ function _select_block_macro(clauses)
end
end
select_kill_rivals(tasks, $i)
$mutate_channel
put!(winner_ch, $(esc(body)))
event_val = $mutate_channel
put!(winner_ch, ($i, event_val))
catch err
throwto(maintask, err)
Base.throwto(maintask, err)
end
end
end
end # if
end # for
push!(branches.args, branch)

body_branch = :(if branch_id == $i; $bind_variable; $(esc(body)); end)
# the next two lines build an if / elseif chain from the bottom up
push!(body_branch.args, body_branches)
body_branches = body_branch
end
quote
winner_ch = Channel(1)
tasks = Array(Task, $(length(clauses)))
maintask = current_task()
$branches
take!(winner_ch)
$branches # set up competing tasks
(branch_id, branch_val) = take!(winner_ch) # get the id of the winning task
$body_branches # execute the winning block in the original lexical context
end
end

Expand Down
7 changes: 2 additions & 5 deletions doc/manual/parallel-computing.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -484,11 +484,8 @@ Consider this simple example which times out if a computation takes too long::
end

@select begin
if output_channel |> value
println("Calculation produced $value")
elseif timeout_channel
println("Timed out waiting for computation")
end
output_channel |> value => println("Calculation produced $value")
timeout_channel => println("Timed out waiting for computation")
end
end

Expand Down
21 changes: 7 additions & 14 deletions test/channels.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -30,13 +30,10 @@ function select_block_test(t1, t2, t3, t4)
end

@select begin
if c1 |> x
"Got $x from c1"
elseif c2
"Got a message from c2"
elseif c3 <| :write_test
"Wrote to c3"
elseif task |> z
c1 |> x => "Got $x from c1"
c2 => "Got a message from c2"
c3 <| :write_test => "Wrote to c3"
task |> z => begin
"Task finished with $z"
end
end
Expand All @@ -59,13 +56,9 @@ function select_nonblock_test(test)
end

@select begin
if c |> x
"Got $x from c"
elseif c2 <| 1
"Wrote to c2"
else
"Default case"
end
c |> x => "Got $x from c"
c2 <| 1 => "Wrote to c2"
_ => "Default case"
end
end

Expand Down