internal/lsp/diff: new diff implementation to replace go-diff

The new implementation is based on Myers' paper, and is in the
package diff/lcs.

There is a new option newDiff, that can be set to 'old', 'new',
or 'both'. The default is 'both', although that may not be
the right choice for a release. See gopls/hooks/diff.go.
'both' runs both the old and new diff algorithm and saves some
statistics in a file in os.Tempdir(),

When (or if) the new code becomes the default, this logging (and
some internal checking) will be removed.

The new implementation has internal checking, which currently
panics. The code in gopls/hooks/diff.go tries to save an encrypted
(for privacy) version of the failing input.

The package diff/myers has not been replaced, but it could be.

Fixes golang/go#52966

Change-Id: Id38d76ed383c4330d9373580561765b5a2412587
Reviewed-on: https://go-review.googlesource.com/c/tools/+/396855
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Peter Weinberger <pjw@google.com>

go.mod

@@ -1,12 +1,12 @@
 module golang.org/x/tools
 
-go 1.17
+go 1.18
 
 require (
 	github.com/yuin/goldmark v1.4.1
 	golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4
 	golang.org/x/net v0.0.0-20211015210444-4f30a5c0130f
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
-	golang.org/x/sys v0.0.0-20211019181941-9d821ace8654
+	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a
 	golang.org/x/text v0.3.7
 )

go.sum

@@ -1,30 +1,12 @@
 github.com/yuin/goldmark v1.4.1 h1:/vn0k+RBvwlxEmP5E7SZMqNxPhfMVFEJiykr15/0XKM=
 github.com/yuin/goldmark v1.4.1/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1Zlc8k=
-golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 h1:6zppjxzCulZykYSLyVDYbneBfbaBIQPYMevg0bEwv2s=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
-golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
-golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/net v0.0.0-20211015210444-4f30a5c0130f h1:OfiFi4JbukWwe3lzw+xunroH1mnC1e2Gy5cxNJApiSY=
 golang.org/x/net v0.0.0-20211015210444-4f30a5c0130f/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
-golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20210220032951-036812b2e83c h1:5KslGYwFpkhGh+Q16bwMP3cOontH8FOep7tGV86Y7SQ=
 golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20211019181941-9d821ace8654 h1:id054HUawV2/6IGm2IV8KZQjqtwAOo2CYlOToYqa0d0=
-golang.org/x/sys v0.0.0-20211019181941-9d821ace8654/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
-golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
-golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
-golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a h1:dGzPydgVsqGcTRVwiLJ1jVbufYwmzD3LfVPLKsKg+0k=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/text v0.3.7 h1:olpwvP2KacW1ZWvsR7uQhoyTYvKAupfQrRGBFM352Gk=
 golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
-golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
-golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

gopls/doc/settings.md

@@ -452,6 +452,16 @@ Default: `false`.
 
 <!-- END User: DO NOT MANUALLY EDIT THIS SECTION -->
 
+#### **newDiff** *string*
+
+newDiff enables the new diff implementation. If this is "both",
+for now both diffs will be run and statistics will be generateted in
+a file in $TMPDIR. This is a risky setting; help in trying it
+is appreciated. If it is "old" the old implementation is used,
+and if it is "new", just the new implementation is used.
+
+Default: 'old'.
+
 ## Code Lenses
 
 These are the code lenses that `gopls` currently supports. They can be enabled

gopls/go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/jba/templatecheck v0.6.0
 	github.com/sergi/go-diff v1.1.0
 	golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4
-	golang.org/x/sys v0.0.0-20220209214540-3681064d5158
+	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a
 	golang.org/x/tools v0.1.11-0.20220523181440-ccb10502d1a5
 	golang.org/x/vuln v0.0.0-20220718121659-b9a3ad919698
 	honnef.co/go/tools v0.3.2
@@ -20,7 +20,7 @@ require (
 	github.com/BurntSushi/toml v1.0.0 // indirect
 	github.com/google/safehtml v0.0.2 // indirect
 	golang.org/x/exp/typeparams v0.0.0-20220218215828-6cf2b201936e // indirect
-	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c // indirect
+	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 	golang.org/x/text v0.3.7 // indirect
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 )

gopls/go.sum

@@ -1,21 +1,14 @@
-github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
-github.com/BurntSushi/toml v0.4.1/go.mod h1:CxXYINrC8qIiEnFrOxCa7Jy5BFHlXnUU2pbicEuybxQ=
 github.com/BurntSushi/toml v1.0.0 h1:dtDWrepsVPfW9H/4y7dDgFc2MBUSeJhlaDtK13CxFlU=
 github.com/BurntSushi/toml v1.0.0/go.mod h1:CxXYINrC8qIiEnFrOxCa7Jy5BFHlXnUU2pbicEuybxQ=
 github.com/client9/misspell v0.3.4 h1:ta993UF76GwbvJcIo3Y68y/M3WxlpEHPWIGDkJYwzJI=
-github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/frankban/quicktest v1.14.2 h1:SPb1KFFmM+ybpEjPUhCCkZOM5xlovT5UbrMvWnXyBns=
 github.com/frankban/quicktest v1.14.2/go.mod h1:mgiwOwqx65TmIk1wJ6Q7wvnVMocbUorkibMOrVTHZps=
-github.com/google/go-cmdtest v0.4.0/go.mod h1:apVn/GCasLZUVpAJ6oWAuyP7Ne7CEsQbTnc0plM3m+o=
-github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
-github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.7 h1:81/ik6ipDQS2aGcBfIN5dHDB36BwrStyeAQquSYCV4o=
 github.com/google/go-cmp v0.5.7/go.mod h1:n+brtR0CgQNWTVd5ZUFpTBC8YFBDLK/h/bpaJ8/DtOE=
-github.com/google/renameio v0.1.0/go.mod h1:KWCgfxg9yswjAJkECMjeO8J8rahYeXnNhOm40UhjYkI=
 github.com/google/safehtml v0.0.2 h1:ZOt2VXg4x24bW0m2jtzAOkhoXV0iM8vNKc0paByCZqM=
 github.com/google/safehtml v0.0.2/go.mod h1:L4KWwDsUJdECRAEpZoBn3O64bQaywRscowZjJAzjHnU=
 github.com/jba/printsrc v0.2.2 h1:9OHK51UT+/iMAEBlQIIXW04qvKyF3/vvLuwW/hL8tDU=
@@ -47,8 +40,6 @@ golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5y
 golang.org/x/exp/typeparams v0.0.0-20220218215828-6cf2b201936e h1:qyrTQ++p1afMkO4DPEeLGq/3oTsdlvdH4vqZUBWzUKM=
 golang.org/x/exp/typeparams v0.0.0-20220218215828-6cf2b201936e/go.mod h1:AbB0pIl9nAr9wVwH+Z2ZpaocVmF5I4GyWCDIsVjR0bk=
 golang.org/x/mod v0.5.1/go.mod h1:5OXOZSfqPIIbmVBIIKWRFfZjPR0E5r58TLhUjH0a2Ro=
-golang.org/x/mod v0.6.0-dev.0.20211013180041-c96bc1413d57/go.mod h1:3p9vT2HGsQu2K1YbXdKPJLVgG5VJdoTa1poYQBtP1AY=
-golang.org/x/mod v0.6.0-dev.0.20220106191415-9b9b3d81d5e3/go.mod h1:3p9vT2HGsQu2K1YbXdKPJLVgG5VJdoTa1poYQBtP1AY=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 h1:6zppjxzCulZykYSLyVDYbneBfbaBIQPYMevg0bEwv2s=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
@@ -61,18 +52,15 @@ golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20211019181941-9d821ace8654/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20211213223007-03aa0b5f6827/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20220209214540-3681064d5158 h1:rm+CHSpPEEW2IsXUib1ThaHIjuBVZjxNgSKmBLFfD4c=
 golang.org/x/sys v0.0.0-20220209214540-3681064d5158/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a h1:dGzPydgVsqGcTRVwiLJ1jVbufYwmzD3LfVPLKsKg+0k=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.7 h1:olpwvP2KacW1ZWvsR7uQhoyTYvKAupfQrRGBFM352Gk=
 golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
-golang.org/x/vuln v0.0.0-20220503210553-a5481fb0c8be h1:jokAF1mfylAi1iTQx7C44B7vyXUcSEMw8eDv0PzNu8s=
-golang.org/x/vuln v0.0.0-20220503210553-a5481fb0c8be/go.mod h1:twca1SxmF6/i2wHY/mj1vLIkkHdp+nil/yA32ZOP4kg=
 golang.org/x/vuln v0.0.0-20220613164644-4eb5ba49563c h1:r5bbIROBQtRRgoutV8Q3sFY58VzHW6jMBYl48ANSyS4=
 golang.org/x/vuln v0.0.0-20220613164644-4eb5ba49563c/go.mod h1:UZshlUPxXeGUM9I14UOawXQg6yosDE9cr1vKY/DzgWo=
 golang.org/x/vuln v0.0.0-20220718121659-b9a3ad919698 h1:9lgpkUgjzoIcZYp7/UPFO/0jIlYcokcEjqWm0hj9pzE=
@@ -88,14 +76,10 @@ gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.4 h1:/eiJrUcujPVeJ3xlSWaiNi3uSVmDGBK1pDHUHAnao1I=
 gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-honnef.co/go/tools v0.2.2/go.mod h1:lPVVZ2BS5TfnjLyizF7o7hv7j9/L+8cZY2hLyjP9cGY=
-honnef.co/go/tools v0.3.0 h1:2LdYUZ7CIxnYgskbUZfY7FPggmqnh6shBqfWa8Tn3XU=
-honnef.co/go/tools v0.3.0/go.mod h1:vlRD9XErLMGT+mDuofSr0mMMquscM/1nQqtRSsh6m70=
 honnef.co/go/tools v0.3.2 h1:ytYb4rOqyp1TSa2EPvNVwtPQJctSELKaMyLfqNP4+34=
 honnef.co/go/tools v0.3.2/go.mod h1:jzwdWgg7Jdq75wlfblQxO4neNaFFSvgc1tD5Wv8U0Yw=
 mvdan.cc/gofumpt v0.3.0 h1:kTojdZo9AcEYbQYhGuLf/zszYthRdhDNDUi2JKTxas4=
 mvdan.cc/gofumpt v0.3.0/go.mod h1:0+VyGZWleeIj5oostkOex+nDBA0eyavuDnDusAJ8ylo=
 mvdan.cc/unparam v0.0.0-20211214103731-d0ef000c54e5 h1:Jh3LAeMt1eGpxomyu3jVkmVZWW2MxZ1qIIV2TZ/nRio=
-mvdan.cc/unparam v0.0.0-20211214103731-d0ef000c54e5/go.mod h1:b8RRCBm0eeiWR8cfN88xeq2G5SG3VKGO+5UPWi5FSOY=
 mvdan.cc/xurls/v2 v2.4.0 h1:tzxjVAj+wSBmDcF6zBB7/myTy3gX9xvi8Tyr28AuQgc=
 mvdan.cc/xurls/v2 v2.4.0/go.mod h1:+GEjq9uNjqs8LQfM9nVnM8rff0OQ5Iash5rzX+N1CSg=

gopls/internal/hooks/diff.go

@@ -5,13 +5,177 @@
 package hooks
 
 import (
+	"crypto/rand"
+	"encoding/json"
 	"fmt"
+	"io"
+	"log"
+	"math/big"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"time"
+	"unicode"
 
 	"github.com/sergi/go-diff/diffmatchpatch"
 	"golang.org/x/tools/internal/lsp/diff"
 	"golang.org/x/tools/internal/span"
 )
 
+// structure for saving information about diffs
+// while the new code is being rolled out
+type diffstat struct {
+	Before, After      int
+	Oldedits, Newedits int
+	Oldtime, Newtime   time.Duration
+	Stack              string
+	Msg                string `json:",omitempty"` // for errors
+	Ignored            int    `json:",omitempty"` // numbr of skipped records with 0 edits
+}
+
+var (
+	mu      sync.Mutex // serializes writes and protects ignored
+	difffd  io.Writer
+	ignored int // lots of the diff calls have 0 diffs
+)
+
+var fileonce sync.Once
+
+func (s *diffstat) save() {
+	// save log records in a file in os.TempDir().
+	// diff is frequently called with identical strings, so
+	// these are somewhat compressed out
+	fileonce.Do(func() {
+		fname := filepath.Join(os.TempDir(), fmt.Sprintf("gopls-diff-%x", os.Getpid()))
+		fd, err := os.Create(fname)
+		if err != nil {
+			// now what?
+		}
+		difffd = fd
+	})
+
+	mu.Lock()
+	defer mu.Unlock()
+	if s.Oldedits == 0 && s.Newedits == 0 {
+		if ignored < 15 {
+			// keep track of repeated instances of no diffs
+			// but only print every 15th
+			ignored++
+			return
+		}
+		s.Ignored = ignored + 1
+	} else {
+		s.Ignored = ignored
+	}
+	ignored = 0
+	// it would be really nice to see why diff was called
+	_, f, l, ok := runtime.Caller(2)
+	if ok {
+		var fname string
+		fname = filepath.Base(f) // diff is only called from a few places
+		s.Stack = fmt.Sprintf("%s:%d", fname, l)
+	}
+	x, err := json.Marshal(s)
+	if err != nil {
+		log.Print(err) // failure to print statistics should not stop gopls
+	}
+	fmt.Fprintf(difffd, "%s\n", x)
+}
+
+// save encrypted versions of the broken input and return the file name
+// (the saved strings will have the same diff behavior as the user's strings)
+func disaster(before, after string) string {
+	// encrypt before and after for privacy. (randomized monoalphabetic cipher)
+	// got will contain the substitution cipher
+	// for the runes in before and after
+	got := map[rune]rune{}
+	for _, r := range before {
+		got[r] = ' ' // value doesn't matter
+	}
+	for _, r := range after {
+		got[r] = ' '
+	}
+	repl := initrepl(len(got))
+	i := 0
+	for k := range got { // randomized
+		got[k] = repl[i]
+		i++
+	}
+	// use got to encrypt before and after
+	subst := func(r rune) rune { return got[r] }
+	first := strings.Map(subst, before)
+	second := strings.Map(subst, after)
+
+	// one failure per session is enough, and more private.
+	// this saves the last one.
+	fname := fmt.Sprintf("%s/gopls-failed-%x", os.TempDir(), os.Getpid())
+	fd, err := os.Create(fname)
+	defer fd.Close()
+	_, err = fd.Write([]byte(fmt.Sprintf("%s\n%s\n", string(first), string(second))))
+	if err != nil {
+		// what do we tell the user?
+		return ""
+	}
+	// ask the user to send us the file, somehow
+	return fname
+}
+
+func initrepl(n int) []rune {
+	repl := make([]rune, 0, n)
+	for r := rune(0); len(repl) < n; r++ {
+		if unicode.IsLetter(r) || unicode.IsNumber(r) {
+			repl = append(repl, r)
+		}
+	}
+	// randomize repl
+	rdr := rand.Reader
+	lim := big.NewInt(int64(len(repl)))
+	for i := 1; i < n; i++ {
+		v, _ := rand.Int(rdr, lim)
+		k := v.Int64()
+		repl[i], repl[k] = repl[k], repl[i]
+	}
+	return repl
+}
+
+// BothDiffs edits calls both the new and old diffs, checks that the new diffs
+// change before into after, and attempts to preserve some statistics.
+func BothDiffs(uri span.URI, before, after string) (edits []diff.TextEdit, err error) {
+	// The new diff code contains a lot of internal checks that panic when they
+	// fail. This code catches the panics, or other failures, tries to save
+	// the failing example (and ut wiykd ask the user to send it back to us, and
+	// changes options.newDiff to 'old', if only we could figure out how.)
+	stat := diffstat{Before: len(before), After: len(after)}
+	now := time.Now()
+	Oldedits, oerr := ComputeEdits(uri, before, after)
+	if oerr != nil {
+		stat.Msg += fmt.Sprintf("old:%v", oerr)
+	}
+	stat.Oldedits = len(Oldedits)
+	stat.Oldtime = time.Since(now)
+	defer func() {
+		if r := recover(); r != nil {
+			disaster(before, after)
+			edits, err = Oldedits, oerr
+		}
+	}()
+	now = time.Now()
+	Newedits, rerr := diff.NComputeEdits(uri, before, after)
+	stat.Newedits = len(Newedits)
+	stat.Newtime = time.Now().Sub(now)
+	got := diff.ApplyEdits(before, Newedits)
+	if got != after {
+		stat.Msg += "FAIL"
+		disaster(before, after)
+		stat.save()
+		return Oldedits, oerr
+	}
+	stat.save()
+	return Newedits, rerr
+}
+
 func ComputeEdits(uri span.URI, before, after string) (edits []diff.TextEdit, err error) {
 	// The go-diff library has an unresolved panic (see golang/go#278774).
 	// TODO(rstambler): Remove the recover once the issue has been fixed

gopls/internal/hooks/diff_test.go

@@ -2,15 +2,56 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package hooks_test
+package hooks
 
 import (
+	"fmt"
+	"io/ioutil"
+	"os"
 	"testing"
+	"unicode/utf8"
 
-	"golang.org/x/tools/gopls/internal/hooks"
 	"golang.org/x/tools/internal/lsp/diff/difftest"
 )
 
 func TestDiff(t *testing.T) {
-	difftest.DiffTest(t, hooks.ComputeEdits)
+	difftest.DiffTest(t, ComputeEdits)
+}
+
+func TestRepl(t *testing.T) {
+	t.Skip("just for checking repl by looking at it")
+	repl := initrepl(800)
+	t.Errorf("%q", string(repl))
+	t.Errorf("%d", len(repl))
+}
+
+func TestDisaster(t *testing.T) {
+	a := "This is a string,(\u0995) just for basic functionality"
+	b := "Ths is another string, (\u0996) to see if disaster will store stuff correctly"
+	fname := disaster(a, b)
+	buf, err := ioutil.ReadFile(fname)
+	if err != nil {
+		t.Errorf("error %v reading %s", err, fname)
+	}
+	var x, y string
+	n, err := fmt.Sscanf(string(buf), "%s\n%s\n", &x, &y)
+	if n != 2 {
+		t.Errorf("got %d, expected 2", n)
+		t.Logf("read %q", string(buf))
+	}
+	if a == x || b == y {
+		t.Error("failed to encrypt")
+	}
+	err = os.Remove(fname)
+	if err != nil {
+		t.Errorf("%v removing %s", err, fname)
+	}
+	alen, blen := utf8.RuneCount([]byte(a)), utf8.RuneCount([]byte(b))
+	xlen, ylen := utf8.RuneCount([]byte(x)), utf8.RuneCount([]byte(y))
+	if alen != xlen {
+		t.Errorf("a; got %d, expected %d", xlen, alen)
+	}
+	if blen != ylen {
+		t.Errorf("b: got %d expected %d", ylen, blen)
+	}
 }