From afdef4345f79d9b091a317aa94701c569391167e Mon Sep 17 00:00:00 2001
From: Jordan Lewis <jordanthelewis@gmail.com>
Date: Fri, 3 May 2019 13:12:31 -0400
Subject: [PATCH] encode: use 64-bit parsing for float32s

Previously, float4 values were incorrectly parsed by this library. For
example, a value of `1.2::float4` would be returned to the user as a
float64 of `1.2000000476837158`. Now, float4s are parsed correctly,
using the 64-bit-float string parsing facility, and return 1.2 as
expected. Note that this problem only showed up if you used the returned
float64 value from lib/pq directly - casting to a float32 would make the
problem stay hidden - but since lib/pq returns float64, we should make
an effort for that value to be correct anyway.
---
 conn_test.go | 8 ++++++--
 encode.go    | 9 ++++-----
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/conn_test.go b/conn_test.go
index 0eba3e5a..5faf1786 100644
--- a/conn_test.go
+++ b/conn_test.go
@@ -1127,10 +1127,11 @@ func TestReadFloatPrecision(t *testing.T) {
 	db := openTestConn(t)
 	defer db.Close()
 
-	row := db.QueryRow("SELECT float4 '0.10000122', float8 '35.03554004971999'")
+	row := db.QueryRow("SELECT float4 '0.10000122', float8 '35.03554004971999', float4 '1.2'")
 	var float4val float32
 	var float8val float64
-	err := row.Scan(&float4val, &float8val)
+	var float4val2 float64
+	err := row.Scan(&float4val, &float8val, &float4val2)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1140,6 +1141,9 @@ func TestReadFloatPrecision(t *testing.T) {
 	if float8val != float64(35.03554004971999) {
 		t.Errorf("Expected float8 fidelity to be maintained; got no match")
 	}
+	if float4val2 != float64(1.2) {
+		t.Errorf("Expected float4 fidelity into a float64 to be maintained; got no match")
+	}
 }
 
 func TestXactMultiStmt(t *testing.T) {
diff --git a/encode.go b/encode.go
index 3b0d365f..a6902fae 100644
--- a/encode.go
+++ b/encode.go
@@ -117,11 +117,10 @@ func textDecode(parameterStatus *parameterStatus, s []byte, typ oid.Oid) interfa
 		}
 		return i
 	case oid.T_float4, oid.T_float8:
-		bits := 64
-		if typ == oid.T_float4 {
-			bits = 32
-		}
-		f, err := strconv.ParseFloat(string(s), bits)
+		// We always use 64 bit parsing, regardless of whether the input text is for
+		// a float4 or float8, because clients expect float64s for all float datatypes
+		// and returning a 32-bit parsed float64 produces lossy results.
+		f, err := strconv.ParseFloat(string(s), 64)
 		if err != nil {
 			errorf("%s", err)
 		}