Selectively reduce multithreaded parsing @error

src/context.jl

@@ -649,9 +649,10 @@ end
             debug && println("single-threaded estimated rows = $origrowsguess, multi-threaded estimated rows = $rowsguess")
             debug && println("multi-threaded column types sampled as: $columns")
         else
-            @error "Multi-threaded parsing failed (are there newlines inside quoted fields?), falling back to single-threaded parsing"
+            # The following debug statement is doubled by a loud @warning or @error in parsefilechunk!
+            debug && println("multi-threaded parsing failed! Falling back to single thread, reinitializing column types.")
             reinitialize_column_type!(columns, types, names, stringtype, streaming)
-            threaded = false
+            threaded = false # the failing is signaled by having !ctx.threaded && ctx.ntasks > 1
         end
     end
     if !threaded

src/file.jl

@@ -564,6 +564,15 @@ function parsefilechunk!(ctx::Context, pos, len, rowsguess, rowoffset, columns,
                 rowsguess = newrowsguess
             end
         end
+        if !ctx.threaded && ctx.ntasks > 1 && !ctx.silencewarnings
+            # !ctx.threaded && ctx.ntasks > 1 indicate that multithreaded parsing failed.
+            # Thes messages echo the corresponding debug statement in the definition of ctx
+            if numwarnings[] > 0
+                @warn "Multithreaded parsing failed and fell back to single-threaded parsing, check previous warnings for possible reasons."
+            else
+                @error "Multithreaded parsing failed and fell back to single-threaded parsing. This can happen if the input contains multi-line fields; otherwise, please report this issue."
+            end
+        end
     end
     # done parsing (at least this chunk), so resize columns to final row count
     for col in columns

test/testfiles.jl

@@ -709,3 +709,16 @@ testfile("test_basic.csv", (types=Dict(2=>Float64),),
     (col1 = [1, 4, 7], col2 = [2.0, 5.0, 8.0], col3 = [3, 6, 9]);
     dir=Path(dir)
 )
+
+# https://github.com/JuliaData/CSV.jl/pull/1099
+@info "The following test is expected to @error with \"Multithreaded parsing failed...\""
+testfile("test_multiline_field_errorwarning.csv", (ntasks=3,),
+    (20, 3),
+    NamedTuple{(:col1, :col2, :col3), Tuple{String3, String, Int}},
+    let col1 = [String3("A$i") for i in 1:19], col2 = [".$i" for i in 1:19], col3 = collect(1:19)
+        insert!(col1, 14, String3("foo"))
+        insert!(col2, 14, "a field to thwart all heuristics\n  ,,,\n, ,\n   , ,,\n, ,,\n  , ,,\n,,\n    ,,\n  ,    ,\n ,   ,\n ,,  ,  ,\n  ,  ,,,\n,   ,,\n\n, , ,    ,\n    ,     ,\n\n  ,,,\n,,,\n,,,\n ,,,\n\n,\n,\n")
+        insert!(col3, 14, 0)
+        (; col1, col2, col3)
+    end
+)

test/testfiles/test_multiline_field_errorwarning.csv

@@ -0,0 +1,45 @@
+col1,col2,col3
+A1,.1,1
+A2,.2,2
+A3,.3,3
+A4,.4,4
+A5,.5,5
+A6,.6,6
+A7,.7,7
+A8,.8,8
+A9,.9,9
+A10,.10,10
+A11,.11,11
+A12,.12,12
+A13,.13,13
+foo,"a field to thwart all heuristics
+  ,,,
+, ,
+   , ,,
+, ,,
+  , ,,
+,,
+    ,,
+  ,    ,
+ ,   ,
+ ,,  ,  ,
+  ,  ,,,
+,   ,,
+
+, , ,    ,
+    ,     ,
+
+  ,,,
+,,,
+,,,
+ ,,,
+
+,
+,
+",-0
+A14,.14,14
+A15,.15,15
+A16,.16,16
+A17,.17,17
+A18,.18,18
+A19,.19,19