Fix case when ipc stream has no record batches, only schema (#175)

* Fix case when ipc stream has no record batches, only schema

Fixes #158. While the Julia implementation currently doesn't provide
way to avoid writing any record batches, the pyarrow implementation has
more fine-grained control over writing and allows closing an ipc stream
without writing any record batches. In that case, on the Julia side when
reading, we just need to check for this case specifically and if so,
populate some empty columns, since we're currently relying on them being
populated when record batches are read.

* fix metadata

src/table.jl

@@ -217,6 +217,7 @@ function Table(bytes::Vector{UInt8}, off::Integer=1, tlen::Union{Integer, Nothin
             i += 1
         end
     end
+    anyrecordbatches = false
     for batch in BatchIterator(bytes, off)
         # store custom_metadata of batch.msg?
         header = batch.msg.header
@@ -258,6 +259,7 @@ function Table(bytes::Vector{UInt8}, off::Integer=1, tlen::Union{Integer, Nothin
             dictencodings[id] = DictEncoding{eltype(A), S, typeof(A)}(id, A, field.dictionary.isOrdered, values.metadata)
             @debug 1 "parsed dictionary batch message: id=$id, data=$values\n"
         elseif header isa Meta.RecordBatch
+            anyrecordbatches = true
             @debug 1 "parsing record batch message: compression = $(header.compression)"
             put!(tsks, Threads.@spawn begin
                 collect(VectorIterator(sch, batch, dictencodings, convert))
@@ -270,13 +272,20 @@ function Table(bytes::Vector{UInt8}, off::Integer=1, tlen::Union{Integer, Nothin
     wait(tsk)
     lu = lookup(t)
     ty = types(t)
+    # 158; some implementations may send 0 record batches
+    if !anyrecordbatches
+        for field in sch.fields
+            T = juliaeltype(field, buildmetadata(field), convert)
+            push!(columns(t), T[])
+        end
+    end
     for (nm, col) in zip(names(t), columns(t))
         lu[nm] = col
         push!(ty, eltype(col))
     end
     meta = sch !== nothing ? sch.custom_metadata : nothing
     if meta !== nothing
-        getfield(t, :metadata)[] = Dict(x.key=>x.value for x in meta)
+        getfield(t, :metadata)[] = buildmetadata(meta)
     end
     return t
 end
@@ -337,6 +346,7 @@ end
 buildmetadata(f::Meta.Field) = buildmetadata(f.custom_metadata)
 buildmetadata(meta) = Dict(String(kv.key) => String(kv.value) for kv in meta)
 buildmetadata(::Nothing) = nothing
+buildmetadata(x::Dict{String, String}) = x
 
 function Base.iterate(x::VectorIterator, (columnidx, nodeidx, bufferidx)=(Int64(1), Int64(1), Int64(1)))
     columnidx > length(x.schema.fields) && return nothing

test/runtests.jl

@@ -295,6 +295,19 @@ c = Arrow.ToTimestamp(x)
 @test eltype(c) == Arrow.Timestamp{Arrow.Flatbuf.TimeUnitModule.MILLISECOND, Symbol("Europe/Paris")}
 @test c[1] == Arrow.Timestamp{Arrow.Flatbuf.TimeUnitModule.MILLISECOND, Symbol("Europe/Paris")}(1577836800000)
 
+# 158
+# arrow ipc stream generated from pyarrow with no record batches
+bytes = UInt8[0xff, 0xff, 0xff, 0xff, 0x78, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0a, 0x00, 0x0c, 0x00,
+    0x06, 0x00, 0x05, 0x00, 0x08, 0x00, 0x0a, 0x00, 0x00, 0x00, 0x00, 0x01, 0x04, 0x00, 0x0c, 0x00, 0x00, 0x00, 0x08, 0x00,
+    0x08, 0x00, 0x00, 0x00, 0x04, 0x00, 0x08, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x14, 0x00,
+    0x00, 0x00, 0x10, 0x00, 0x14, 0x00, 0x08, 0x00, 0x06, 0x00, 0x07, 0x00, 0x0c, 0x00, 0x00, 0x00, 0x10, 0x00, 0x10, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x01, 0x02, 0x10, 0x00, 0x00, 0x00, 0x1c, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x61, 0x00, 0x00, 0x00, 0x08, 0x00, 0x0c, 0x00, 0x08, 0x00, 0x07, 0x00, 0x08, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x40, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00]
+tbl = Arrow.Table(bytes)
+@test length(tbl.a) == 0
+@test eltype(tbl.a) == Union{Int64, Missing}
+
 end # @testset "misc"
 
 end