Not consistently, but once every 10 tries or so, I get the following backtrace which does not seem to originate in my own code:
Can not modify the buffer right now, there are active subbuffers. -:1: _static_modules._Stdio()->Buffer(0 bytes, read=[..-1] data=[0..-1] free=[0.. 5300] allocated (read only))->add("\f\0\0n\3\0\35 \237{å\32B«=B/ûÓ\2"DmøkFñ 3\35¤\3\37Ò\214+ÜÔ*\4\3\0F0D\2 =Í3]r¿\20CN\222²`ÞG"Ы·\23aÆ\r\237{q)Ùß\t\ 206w(\2 {\23$Ò\4vì\b\227ú\31n\24xÈ¢M\v8d F\1/z\211\vÈx¹ï\222") /usr/local/pike/8.1.13/lib/modules/SSL.pmod/Connection.pike:1051: SSL.ClientConnection(handshaking)->got_data("\26\3\3\0;\2\0\u00007\3\3\ÿP¹\ 17êlU:b5ñwÖäO»àz£eæ(§DOWNGRD\1\0À+\0\0\17ÿ\1\0\1\0\0\v\0\2\1\0\0#\0\0\26\3\3 \r8\v\0\r4\0\r1\0\bË0\202\bÇ0\202\a¯ \3\2\1\2\2\209Î\222 tü½:O5Øj\21ö\rh0\r\ 6\t*\206H\206÷\r\1\1\v\5\u00000T1\v0\t\6\3U\4\6\23\2US1\u001e0\34\6\3"+[3389 ]+"\206w(\2 {\23$Ò\4vì\b\227ú\31n\24xÈ¢M\v8d F\1/z\211\vÈx¹ï\222\26\3\3\0\4\ 16\0\0\0") /usr/local/pike/8.1.13/lib/modules/SSL.pmod/File.pike:1985: SSL.File(Stdio.File("socket", "172.217.20.106:443", 777 /* fd=43 */), SSL.Cl ientConnection(handshaking))->ssl_read_callback(Stdio.File("socket", "172.21 7.20.106:443", 777 /* fd=43 */),"\26\3\3\0;\2\0\u00007\3\3\ÿP¹\17êlU:b5ñwÖä O»àz£eæ(§DOWNGRD\1\0À+\0\0\17ÿ\1\0\1\0\0\v\0\2\1\0\0#\0\0\26\3\3\r8\v\0\r4\0 \r1\0\bË0\202\bÇ0\202\a¯ \3\2\1\2\2\209Î\222 tü½"+[3448]+"ú\31n\24xÈ¢M\v8d F \1/z\211\vÈx¹ï\222\26\3\3\0\4\16\0\0\0") /usr/local/pike/8.1.13/lib/modules/Stdio.pmod/module.pmod:1165: Stdio.File("socket", "172.217.20.106:443", 777 /* fd=43 */)->__stdio_read_ca llback() -:1: Pike.Backend(0)->`()(3600.0) Concurrent.Promise(pending,0): Promise broken. /usr/local/pike/8.1.13/lib/modules/Concurrent.pmod:1176: unknown function() -:1: _static_modules._Stdio()->Buffer(0 bytes, read=[..1279] data=[1280..1279] fr ee=[1280..0] subbuffer)->read_int8() /usr/local/pike/8.1.13/lib/modules/Standards.pmod/ASN1.pmod/Decode.pmod:68: Standards.ASN1.Decode.read_identifier(_static_modules._Stdio()->Buffer(0 byt es, read=[..1279] data=[1280..1279] free=[1280..0] subbuffer)) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/ASN1.pmod/Decode.pmod:100: Standards.ASN1.Decode.der_decode(@0=_static_modules._Stdio()->Buffer(0 bytes , read=[..1279] data=[1280..1279] free=[1280..0] subbuffer),mapping[23]) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/ASN1.pmod/Decode.pmod:157: Standards.ASN1.Decode.der_decode(@0,@1=mapping[23]) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/ASN1.pmod/Decode.pmod:245: Standards.ASN1.Decode.secure_der_decode("0\202\5\0\202\20*.googleapis.com\20 2\24*.clients6.google.ae\202\24*.clients6.google.at\202\24*.clients6.google. be\202\24*.clients6.google.ca\202\24*.clients6.google.ch\202\24*.clients6"+[ 1094]+"is.com\202\26cloudendpointsapis.com\202\16googleapis.com",@1) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/X509.pmod:703: Standards.X509.TBSCertificate(([ /* 5 elements */ "algorithm": Standards.ASN1.Types.Sequence(({ /* 2 elements */ Standards.ASN1.Types.Identifier(1.2.840.113549.1.1.11), Standards.ASN1.Types.Null() })), "issuer": ([ /* 3 elements */ "commonName": "Google Internet Authority G3", "countryName": "US", "organizationName": "Google Trust Services" ]), "serial": Gmp.mpz(76838582857112318721873235160560176488), "subject": ([ /* 5 elements */ "commonName": "*.googleapis.com", "countryName": "US", "localityName": "Mountain View", "organizationName": "Google LLC", "stateOrProvinceName": "California" ]), "version": 3 ]))->`raw_extensions=(Standards.ASN1.Types.Sequence(...)) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/X509.pmod:938: Standards.X509.TBSCertificate(([ /* 5 elements */ "algorithm": Standards.ASN1.Types.Sequence(({ /* 2 elements */ Standards.ASN1.Types.Identifier(1.2.840.113549.1.1.11), Standards.ASN1.Types.Null() })), "issuer": ([ /* 3 elements */ "commonName": "Google Internet Authority G3", "countryName": "US", "organizationName": "Google Trust Services" ]), "serial": Gmp.mpz(76838582857112318721873235160560176488), "subject": ([ /* 5 elements */ "commonName": "*.googleapis.com", "countryName": "US", "localityName": "Mountain View", "organizationName": "Google LLC", "stateOrProvinceName": "California" ]), "version": 3 ]))->init(Standards.ASN1.Types.Sequence(...)) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/X509.pmod:763: Standards.X509.TBSCertificate(([ /* 5 elements */ "algorithm": Standards.ASN1.Types.Sequence(({ /* 2 elements */ Standards.ASN1.Types.Identifier(1.2.840.113549.1.1.11), Standards.ASN1.Types.Null() })), "issuer": ([ /* 3 elements */ "commonName": "Google Internet Authority G3", "countryName": "US", "organizationName": "Google Trust Services" ]), "serial": Gmp.mpz(76838582857112318721873235160560176488), "subject": ([ /* 5 elements */ "commonName": "*.googleapis.com", "countryName": "US", "localityName": "Mountain View", "organizationName": "Google LLC", "stateOrProvinceName": "California" ]), "version": 3 ]))->create(@2=Standards.ASN1.Types.Sequence(...)) /usr/local/pike/8.1.13/lib/modules/Standards.pmod/X509.pmod:329: Standards.X509.TBSCertificate() /usr/local/pike/8.1.13/lib/modules/Standards.pmod/X509.pmod:1448: Standards.X509.decode_certificate(Standards.PKCS.Signature.Signed(...)) /usr/local/pike/8.1.13/lib/modules/SSL.pmod/Connection.pike:283: SSL.ClientConnection(handshaking)->verify_certificate_chain(({"0\202\bÇ0\202 \a¯ \3\2\1\2\2\209Î\222 tü½:O5Øj\21ö\rh0\r\6\t*\206H\206÷\r\1\1\v\5\u00000T1 \v0\t\6\3U\4\6\23\2US1\u001e0\34\6\3U\4\n"+[2160]+"É\vèÍ|ÂPD\201\220.R#Z³\23 7iG(#ç","0\202\4\0\202\3D \3\2\1\2\2\r\1ã©0\34ür\68?\232S\u001d0\r\6\t*\206 H\206÷\r\1\1\v\5\u00000L1 0\36\6\3U\4\v\23\27GlobalSign Roo"+[1029]+"\234@èÖ Usº<ß\220Ë\0[!\21gÂí2\36Þ"})) /usr/local/pike/8.1.13/lib/modules/SSL.pmod/Connection.pike:372: SSL.ClientConnection(handshaking)->handle_certificates(SSL.Buffer(0 bytes, r ead=[..3379] data=[3380..3379] free=[3380..3480] allocated)) /usr/local/pike/8.1.13/lib/modules/SSL.pmod/ClientConnection.pike:876: SSL.ClientConnection(handshaking)->handle_handshake(11,@3=SSL.Buffer(0 bytes , read=[..3379] data=[3380..3379] free=[3380..3480] allocated),_static_modul es._Stdio()->Buffer(3384 bytes, read=[..-1] data=[0..3383] free=[3384..0] su bbuffer)) /usr/local/pike/8.1.13/lib/modules/SSL.pmod/Connection.pike:1070: SSL.ClientConnection(handshaking)->got_data("\26\3\3\0;\2\0\u00007\3\3\ÿP¹\ 17êlU:b5ñwÖäO»àz£eæ(§DOWNGRD\1\0À+\0\0\17ÿ\1\0\1\0\0\v\0\2\1\0\0#\0\0\26\3\3 \r8\v\0\r4\0\r1\0\bË0\202\bÇ0\202\a¯ \3\2\1\2\2\209Î\222 tü½:O5Øj\21ö\rh0\r\ 6\t*\206H\206÷\r\1\1\v\5\u00000T1\v0\t\6\3U\4\6\23\2US1\u001e0\34\6\3"+[3389 ]+"\206w(\2 {\23$Ò\4vì\b\227ú\31n\24xÈ¢M\v8d F\1/z\211\vÈx¹ï\222\26\3\3\0\4\ 16\0\0\0") /usr/local/pike/8.1.13/lib/modules/SSL.pmod/File.pike:1985: SSL.File(Stdio.File("socket", "172.217.20.106:443", 777 /* fd=43 */), SSL.Cl ientConnection(handshaking))->ssl_read_callback(Stdio.File("socket", "172.21 7.20.106:443", 777 /* fd=43 */),"\26\3\3\0;\2\0\u00007\3\3\ÿP¹\17êlU:b5ñwÖä O»àz£eæ(§DOWNGRD\1\0À+\0\0\17ÿ\1\0\1\0\0\v\0\2\1\0\0#\0\0\26\3\3\r8\v\0\r4\0 \r1\0\bË0\202\bÇ0\202\a¯ \3\2\1\2\2\209Î\222 tü½"+[3448]+"ú\31n\24xÈ¢M\v8d F \1/z\211\vÈx¹ï\222\26\3\3\0\4\16\0\0\0") /usr/local/pike/8.1.13/lib/modules/Stdio.pmod/module.pmod:1165: Stdio.File("socket", "172.217.20.106:443", 777 /* fd=43 */)->__stdio_read_ca llback() -:1: Pike.Backend(0)->`()(3600.0)